    [Plotting Boards M3 and M4]
    [From the U.S. Government Publishing Office, www.gpo.gov]

    ﻿40 /. 35:7 - I Zs.	™ 9-2681
    r-1	■■■—  	—■
    Document	WAR DEPARTMENT
    Reserve ___________________________
    TECHNICAL MANUAL
    INSTRUCTION GUIDE
    PLOTTING BOARDS M3 AND M4
    January 21, 1942
    NON-CIRCULATING
    NTSU LIBRARY
    *TM 9-2681
    1—2
    TECHNICAL MANUAL No. 9-2681
    WAR DEPARTMENT, Washington, January 21, 1942.
    INSTRUCTION GUIDE
    PLOTTING BOARDS M3 AND M4
    Prepared under direction of the Chief of Ordnance
    Paragraph
    General___________________________________________________________________ 1
    Design and use------------------------------------------------------------ 2
    Description of major units________________________________________________ 3
    Operation_______________________________________________________________   4
    Adjustment_______________________________________________________________  5
    Packing and assembling____________________________________________________ 6
    Care and preservation_____________________________________________________ 7
    Page
    Appendix. List of references_____________________________________________ 24
    1. General.—a. Purpose.—This manual is published for the information of the using arms and services.
    5. Scope.—This manual covers the underlying theory of the plotting boards M3 and M4, a detailed description of their parts, complete operating instructions, and necessary care and adjustments to be used for instructional purposes. Adjustments which may be performed by using arms personnel are described.
    2. Design and use.—a. General.—(1) The plotting boards M3 and M4 are designed for use with all types of fixed seacoast artillery cannon. They provide a means for locating the target by either the two-station (horizontal base) or the one-station (vertical or self-contained base) method. The methods of locating and relocating the set-forward point in range and azimuth and determining the angular travel are the same as those employed with the 110° plotting board M1915.
    (2) The plotting boards M3 and M4 differ essentially from the 110° board in that they permit continuous plotting through a field of about 200°, the center line of which corresponds approximately in azimuth with the center line of the field of fire of the gun; are equipped with optical station arms in addition to the mechanical arms; and have each station located only in one quadrant.
    Z>. Comparison.—(1) The plotting board M3 is constructed to a scale of 600 yards per inch, has a plotting radius of 58i/3 inches, and the station and gun arms are graduated from 3,000 to 35,000 yards. The
    *This manual supersedes TM 9—2681, July 10, 1941.
    433584°— 42--1
    1
    TM 9-2681
    2-3
    ORDNANCE DEPARTMENT
    plotting board M4 is constructed to a scale of 800 yards per inch, has a plotting radius of 67^ inches, and the station and gun arms are graduated from 4,000 to 54,000 yards.
    (	2) Inasmuch as the boards differ only in plotting radius and range, the instructions apply to both boards unless otherwise noted.
    3	. Description of major units.—The plotting boards M3 and M4 may be divided into the following units to facilitate description: plot-ing board and azimuth circle, directing point and gun arm, station
    r	. .	..." .	• . .	. ...: - -	-
    23	I	2	4	16 10
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    sA 24	17 16	3	®
    1-	Gun arm.	10. Indicator.
    2.	Gun arm bracket.	16. Azimuth circle.
    3. Mechanical station arm.
    4. Optical station arm.
    6. Index box.
    17. Elbow telescope.
    23. Dust cap.
    24. Leg base.
    Figure 1.—Plotting board M3.
    centers, mechanical station arms and index boxes, optical station arms, indicator and mirror drive, and miscellaneous equipment.
    a.	Plotting board and azimuth circle.— (1) Plotting board proper.— The plotting board consists of a built-up board mounted on a rigid spider, or frame, supported on pipe legs. The legs are provided with jackscrew bases (24, fig. 1) to facilitate leveling the board and to compensate for unevenness in the floor of the plotting room.
    (2)	Azimuth circle.—An azimuth circle (16, fig. 1) is engraved on metal strips which are dovetailed into a series of segments bolted to the outer end of the spider arms. The circle is engraved and num
    2
    TM 9-2681
    3
    PLOTTING BOARDS M3 AND M4
    bered at 1° intervals, and the mean azimuth of the gun is indicated by the graduation on the center line of the board.
    b.	Directing point and gun arm.—The directing point is located at • the center of the azimuth circle, and the gun arm (1, fig. 1) is pivoted in the bracket (2) above this center. The gun arm is also hinged (sec. B-B, fig. 4) on a horizontal pivot to facilitate operation. A subscale (15. fig. 11) is attached to the outer extremity of the gun arm to permit reading gun azimuth to 0.05°.
    c.	Station centers.— (1) Mechanical arm centers.— (a) The base and stations are represented by sleeves, or bushings (sec. D-D, fig. 4), accurately located in azimuth and distance from the directing point. The sleeves used in conjunction with the mechanical station arms are equipped with antifriction bearings into which the pivots of the appropriate station arms are fitted for operation.
    (6) Special station center plugs (sec. C-C, fig. 4) are provided for use in the event that stations are situated so close together that the customary station sleeves cannot be located without mutual interference. (See fig. 2.) Each plug is provided with an antifriction bearing for the station arm pivot and has a square boss machined on its under side. The pivot bearing is located eccentrically with respect to the boss, which provides the means for orienting the station center in either of two positions. It is essential that the coupler corresponding to the position employed be used. Failure to do this will limit the rotation of the mechanical station arm and introduce an error in tracking. An arrow, or index, is engraved on the upper surface of the plug. In use, the boss is placed in a square bushing in the station plate with the arrow pointing toward the identification mark of the station to be employed.
    (2) Optical station centers.—The hollow pivots on the optical arms fit directly into the sleeves of the optical stations. The sleeves in these stations are machined in the upper end of the housings which contain the worm gearing for driving the mirror assemblies. (See sec. A-A. fig. 9.)
    d. Mechanical station arms and index boxes.— (1) The mechanical station arms (3, fig. 11) are equipped with couplers (5), equal in length to the displacement of their respective stations from the directing point. The couplers are attached to index boxes (6) and serve to keep the station arms at all times parallel to imaginary auxiliary arms pivoted at the directing point. An arm is supplied for each mechanical station and the arms are not interchangeable between stations.
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    TM 9-2681
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    ORDNANCE DEPARTMENT
    (2) The index boxes ride along the azimuth circle and are provided with subscales for reading the azimuths of the station arms to 0.05°. Two subscales are provided in each index box, one engraved and filled in with white paint, the other engraved and filled in with red paint.
    AZIMUTH OF NORMAL-TO
    STRIP-C77934---------------
    ARM, GUN, ASSEMBLY-D43499 —
    ARM,OPTICAL, ASSEMBLY-D435O2
    SEGMENT-D435I2
    STRIP-BI39390
    BRACKET-C77936
    INDICATOR, ASSEMBLY-D43508B
    -TUBE-AI8O956B
    -PIN-BFCXIM
    - BOARD-D43496
    PLATE - D435I8--------
    PLUG, ASSEMBLY-BI39395
    BUSHING-AI8O959 ------
    BUSHING-AI8O958-------
    C/i ,78
    Figure 2.—Plotting board
    TELESCOPE,ELBOW.M 10 91-91-1
    DRAWER, ASSEMBLY
    D43525
    SCREW-BCQX2BF
    ARM-D435I5
    The white subscale is for normal use and indicates the true azimuth of the station arm. The red subscale is used only when the station arm or coupler covers the white subscale; it indicates an azimuth 3° greater or less than true azimuth, depending upon which index box is being used. Each index box is equipped with a spring-actuated
    4
    TM 9-2681
    3
    PLOTTING BOARDS M3 AND M4
    latch (7, fig. 11) for locking the couplers, and a lever (8) for releasing the clamping shoe and thereby permitting the box to slide along the azimuth circle.
    e. Optical station arms.— (1) Optical station arms (4, fig. 1) are provided to eliminate the inconvenience which would attend the use
    BOX, RIGHT, ASSEMBLY- D43504 COUPLER, ASSEMBLY-C7793O ARM, STATION, ASSEMBLY-D43500
    SCREW-BCGX3DD
    STRIP-8139389 —
    BOLT-BCAXICD-NUT-BBAXIC — WASHER-BEAXIL-
    CLIP-A48829
    SCREW-BCQX2EK
    ARM-D435I7
    SEGMENT-D435I3
    MIRROR AND ADAPTER, ASSEMBLY-BI376I7
    BOX, LEFT, ASSEMBLY-D43506
    PIN-BFDX5.IE
    —ARM-D435I6 .
    BOLT—4-20x41
    - PIN-BFDXIEL WASHER-BE AXIH INDICATOR, ASSEMBLY-043497
    BRACKET-C78420
    COUPLING, ASSEMBLY- BI73966
    TUBE-AI80956A
    ---SEGMENT-D435II
    PIN-BFDXIEM	rauiM
    BRACK ET-C78420
    COVER-AI79226 J ARM-D435I4---------
    ----SCREW-BCQXIEC ----PLATE-AI8O957
    INDICATOR.ASSEMBLY-D435O8B
    BOLT- 2j-2OX U-]---------
    WASHER-BEAXIH-1
    TUBE-AI8O956C------------
    M4, assembled view.
    of the conventional coupler assembly with stations located 25 inches or more from the directing point on the board, in which case the overhang of the long couplers would interfere with the operating personnel.
    5
    TM 9-2681
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    ORDNANCE DEPARTMENT
    (2) The number of optical arms supplied with each board depends on the number and location of the stations. The arms are interchangeable among the stations, so in no case will more than two
    r—AZIMUTH
    BOX,RIGHT, ASSEMBLY-029224
    ARM,GUN, ASSEMBLY D433I9
    BOARD-D43556
    Figure 3.—Plotting board
    -SPIDER-043539 PLATE-A48830 SCREW-BCQXIEC-PLATE-D43557-----
    ARM, STATION, ASSEMBLY-D43554 TELESCOPE, ELBOW,MIOAI-----
    9I-9HI
    INDICATOR ASSEMBLY — D43508A
    COVER-AI79226
    BOLT-4 -20X&
    WASHER-BEAXIH
    PIN-4-xf------:----
    ARM-D29I56------
    MIRROR, ASSEMBLY
    BI72063
    TUBE, ASSEMBLY— BI72055 PIN-BFCXIM----
    TUBE, ASSEMBLY — BI72056
    BRACKET-C77999— BRACKET-C78000 — ARM-D43553-----
    BOLT- BCAXICD — i NUT-BBAXIC
    WASHER-BEAXIL—J SEGMENT-D4355I — BRACKET-C78006-PIN-BFDXIEL STRIP-C7800I ARM-D43558
    BOLT-BCAXICE-----
    WASHER-BEAXIL
    BRACKET-C78005
    PIN-BFDXIES---
    BOLT -1 - 20 X jg-PIN-BFDXIEL WASHER-BEAXIH
    standard arms be needed. However, on certain boards stations have been located in front of the directing point (fig. 3), and in these cases special arms are supplied which are not interchangeable with the standard arms.
    6
    TM 9-2681
    3
    PLOTTING BOARDS M3 AND M4
    (3) The elbow telescope MIO is used with the standard arm and the M10A1 telescope with the special arm. The two telescopes are identical except that in the M10A1 instrument the image distance
    COUPLER, ASSEMBLY-C78OO3
    ARM, STATION, ASSEMBLY D43329
    BOX, LEFT, ASSEMBLY-D29222
    OF NORMAL-226“
    (-CLIP-A48829
    SCREW-BCQX2EK
    - SEGMENT-D29I59
    -STRIP-078002
    -ARM,STATION, ASSEMBLY-D43238
    -PIN - BFDX5.IE
    -TELESCOPE, ELBOW, MIO 91—91—I
    DRAWER, ASSEMBLY-D43226
    SCREW-BCQX2BF
    SEGMENT-D43552
    INDICATOR, ASSEMBLY-D43508B
    MIRROR AND ADAPTER, ASSEMBLY-BI376I7
    ARM-D29I57
    TUBE-AI8I209B
    — COUPLING,ASSEMBLY-BI73966
    — BRACKET-078004
    — INDICATOR, ASSEMBLY-D43497
    — STRIP-BI72O62
    —SCREW-BCGX3DD
    FRAII43
    M3, assembled view.
    has been increased (fig. 6) to accommodate the decreased object distance employed with the special arm. The front end of the telescope is so constructed (fig. 5) that the sleeve (B173790) can be turned out a sufficient distance to effect this change.
    7
    TM 9-2681
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    ORDNANCE DEPARTMENT
    8
    [—plug.center station,assembly
    -----SCREW —BCUXICB
    r—C0VER-AI79782	i----RING-A48802
    I r SEPARATOR-A4880I r-COVER-AI79413
    M g—---------------NUT-BBSX4A8	, z /z/ zIa-z./J.- z	___r tI—4~77T7TZl_
    C*wLL3----------------washer-AI7922I	[\. 'ifeEr ,/A JR. ffer1
    -	-----------BEARING,RADIAL BALL	>/»	RfJ?
    --------------------SCREW-BCUXIEE	n t ■ ■ > A -kxd tBxvSj --------------------PLUG—AI79223	I/II X'/-? v/‘Etuj-------SHAFT AI79224	SCREW —BCLX3FF—I	SLEEVE—A48800 ; /
    SPRING-A488O3-----' I
    3 v 3	BEARING,RADIAL BALL-1
    PINX -g	.3543X10236 X 453-.437
    NUT—A48804--------
    SECTION B-B	SECTION C-C	SECTION D-D
    -----------WASHER-A488I9 -----------NUT - A48821
    ffiWOW™ ----------------------RACE-A488I8
    ---------NUT-A48820
    --------WASHER -A488I7
    I W ^L.SZ I - - 1. X.	FRAIMC
    WASHER-A488I6 —* ------------------BALL-jg DIAM.
    SECTION E-E
    Figure 4.—Plotting boards M3 and M4, sectional views.
    TM 9-2681
    3
    PLOTTING BOARDS M3 AND M4
    _______ z+4	|---SCREW-BCUXICC
    [SCREW-BCLX3BB pSCREW-BCUXICD
    J—*--------hr
    ♦ J*	J iW<22--------BODY-C70I50
    —41----------------g____\ y "	--VI--WEDGE-A3782I
    V I *	V\ tn	T1--H0LDER-A3782O
    4/	W 14-----PRISM-A36234
    N
    J------1	ADAPTER-6173789	'A-------Y<
    %	Z	SLEEVE-BI73790—	/	•>
    /	\	--CELL-AI84440	<	>
    --------J	---CAP-BI7379I	-/	Z
    ----CAP, ASSEMBLY-8173792	z	/
    1-----0BJECTIVE-A33906 '	'/, t z'
    ---------------------------STRIP-AI84439-•>----7 ,_i_.----------------------RING-A33AIO----	z---	I '/ SCREW-BCUX3AA
    —----------—-Zl_ I | \// SCREW-BCUXICC
    / z«x \	RETICLE-AI79I83- 15^ Z/ SCREW-BCUXICB
    B h	-	\	I 8	RETICLE, ASSEMBLY-8173134	~*L	7/~ ^^'E.C^^?EMBLY
    V ----------1 VW Jr n	CELL-AI8I377---T	J, P // 'l-ENS-ASS9^
    \	)	RING-A339II-----1 ;	—/'/ sealing compound
    _U__|______ \	etching on this side-t' T*- —	/ —
    —	SEPARATOR-A339I6-1	-	‘‘i »  -screw nri iviro
    SEPARAT0R-A339I7-CELL A3S1A
    SHIELD-A39I25-	-lInS-A^H
    EYEt;ANS3-A339^—■	SCREW-BCUXICB
    ।	. ^SCREW-BCUXICC	4.
    ~hz; < z’ 7 7-r\	\
    >jz0	y
    J, V	SECTION B-B
    \ X I	__ |	rSAJHT
    C0VER-A339O4-	--PAD-A3282O
    SECTION A-A
    Figuke 5.—Elbow telescope MIO, assembled and sectional views.
    433584°—42---2
    9
    TM 9-2681
    3
    ORDNANCE DEPARTMENT
    FOR MIO - A'2.330, B=2.30l FOR MIOAI-A'2.552, 8=2.523
    p------------A---------------"I ,
    I' \*	—-K	! --------------------PRISM, AMICI- @6234
    |	X \	APERTURE (FRONT SURFACE)'.62
    ----------------------------------->	--------F	APERTURE (REAR SURFACE)*^? M----------------------------------d J
    OBJECTIVE- @3906----------- I"	8
    APERTURE'1.09	------------------RETICLE - @79183
    APERTURE = .735
    in rO ’T OJ O O
    4- I
    8	.---U—_______________________ETCHING ON THIS SIDE
    J ’__I* FT -lu ijil Ul
    -----|——----------------------------LENS, FIELD - @3913
    £	$	g	APERTURE = 866
    |	; ----------LENS. COLLECTIVE- @3914
    APERTURE*.866
    8	—3—t------------EYELENS- @3915
    ____________I	APERTURE-.732 I
    Power, 8X.	E. F. L. of objective, 4.8.	Diameter of exit pupil, .136.
    Field of view, 8°45'.	E. F. L. of eyepiece, .602.
    Figure 6.—Elbow telescope M10, M10A1, optical system.
    10
    TH 9-2681
    3
    PLOTTING BOARDS M3 AND M4
    (4)	The elbow telescope is positioned accurately on the arm. A cap (B173791), which fits over the objective cell (A184440), supports a narrow vertical metal strip, the front surface of which is painted white. In use, the arm is positioned with respect to the mirror (fig. 7) until the vertical black line on the reticle appears to be superimposed upon the reflected image of the broad white line on the strip across the objective. This condition will prevail only when the line of sight is normal to the mirror. In this position the fiducial edge of the arm will be set to the azimuth indicated on the indicator (10. fig. 13), which is connected to the mirror by means of the tube (19) and worm and gear shown in figure 9.
    - # : , । MIRROR
    WR DRIVE USING
    Figure 7.—Mirror assembly.
    (5)	The elbow telescope MIO (fig. 5) is an 8-power erect-image instrument having a field of view of 8°45' and an exit pupil diameter of 0.136 inch. The optical system (fig. 6) consists of an objective, Amici prism, reticle, and eyepiece assembly made up of an eyelens, field lens and collective lens.
    f. Indicator and mirror drive.—Outside and sectional views of the azimuth indicator and mirror drive mechanisms are shown in figures 8, 9, and 13. Identical units are used on the plotting boards M3 and M4.
    (1) Azimuth indicator.—Referring to figure 8, the indicator consists essentially of a fine and a coarse dial which are interconnected
    11
    TM 9-2681
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    ORDNANCE DEPARTMENT
    by means of suitable gearing and driven by a handwheel to indicate azimuth against a fixed index. The fine dial is pinned directly to the handwheel shaft, is graduated from 0° to 10° at 0.05° intervals, and is numbered every 0.50°. The coarse dial is graduated and numbered from 0° to 360° at 10° intervals. It is mounted on an anti-
    g-| te-co 	4724 X 1.2598 X .500
    KEY-BGAX4C--------------------—1	Fk
    t-----------------—COVER-077932
    BEARING.RADIAL BALL------------ C3==l S
    4XiXS	R LsfV	S	------STUD—A36367
    PINION—AI79I6I---------'	:	>	&	1 - ——HANDLE-A36364
    PIN-BFCXIB---------------------< M
    p—g
    SCREW-BCGX30F-r---------■	PS jfflh vX
    WASHER-BEAXID-1 II II	p -------O	4^44~'--------I----NUT-BBDXIA
    I....“  ----|~Qvv v —rr—	_ji—-----------------WASHER-BEAXIH
    BEARING.RADIAL BALL---------JStPz ''A	® /P OPlVlT-
    3937 XI.I8IIX 354	M
    I	lb M --------1-------SHAFT-6139388
    PIN-BFDXIBG-------------1	7. W-T/X	$
    BRACKET-6139393------------------------I . |	.--------------HANDWHEEL-8138661
    GEAR—AI79I60----------------X '	S
    ■/	----SCREW-BCCXIAA
    SPIDER—AI79I64--------------
    GEAR-AI79I63----------------^1777/77.	'	--------BE ARING.fWDIAL BALL
    PIN—BFDXIBE--------------------------1	' 42	2 X 'i X
    FRAII4I
    SECTION A-A
    Figure 8.—Indicator assembly, counter and handwheel drive.
    friction bearing on the handwheel shaft and is geared to rotate at the rate of 10° per revolution of the hand wheel. The coupler (12, fig- 13) is keyed and pinned to the handwheel shaft and provides the means for adjusting the position of the mirror (fig. 7).
    (2) Mirror drive.—The mirror drive mechanism is connected to the azimuth indicator by means of the tube (19) and coupler (12)
    12
    TM 9-2681
    3-4
    PLOTTING BOARDS M3 AND M4
    as shown in figure 13. The motion of the handwheel is therefore transmitted to the mirror, which is mounted on the upper extremity of the vertical shaft (B139380, fig. 9) in the housing (D43510). The mirror assembly is positioned on the shaft by means of a key (A179177) on the shaft and a key way in the assembly.
    g. Miscellaneous.—(1) The collimating targ (fig. 10) is provided for use in checking and collimating the optical station arm.
    (2)	A small drawer (fig. 14) is built into the board for holding the targs, mirrors, etc., when they are not in use.
    (3)	A packing chest is provided for storing the station arms and station arm couplers when they are not in use.
    4.	Operation.—a. Comparison.— (1) The operation of the plotting board M4 is the same as for the M3 board, since they are similar, except that the M4 board has been designed to accommodate the increased range of modern cannon.
    (2)	In general, the M3 and M4 boards resemble all other standard plotting boards in that they are devices for the graphical representation to a convenient scale of a constantly changing triangle, one side of which is fixed as to length and azimuth. The purpose is to locate the successive positions of the moving vertex of the triangle in the field of operation. The two station arms, or the changing sides of the triangle, represent the line of sight from the observing stations to the target, and the point of intersection of the arms, or the vertex of the triangle, represents tfie position of the target in the field of operation.
    (3)	When the single station method of tracking is used, the target is located in the field of operation by its distance from, and azimuth with respect to, one point.
    (4)	These boards have no special device for the determination of angular travel, such as the gun arm azimuth circle on the Whistler-Hearn type plotting board M1904. Other plotting room devices have been adapted to the use of azimuths of successive plotted points in determining the angular travel of the target for case II pointing. Therefore, the operation of the boards is identical for case II and case III pointing and is similar to the operation of the 110° board and to the operation of the Whistler-Hearn board for case III pointing.
    (5)	For horizontal base tracking with prediction scale and setforward device the personnel usually consists of the plotter, two armsetters, and a set-forward device operator. The azimuth of the set-forward point is read by one of the armsetters. For vertical or self-contained base tracking, only one armsetter is required, and
    13
    TM 9-2681
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    ORDNANCE DEPARTMENT
    when predicting by means of set-forward scales, the set-forward device operator is not needed, since the plotter locates the set-forward point.
    b. Horizontal base.— (1) Setting up.— (a) Remove the dust caps (fig. 10) from the station sleeves being used. Select the proper arms from the packing chest and insert the pivots into their respective station sleeves. For this discussion it will be assumed that one arm is a mechanical arm and the other an optical arm. However,
    I I
    KEY- AI79I77------------------------p —,	<■____
    / /
    HOUSING - D435I0----------------------
    SHAFT - BI39380 ----------------------£ — -	;
    BEARING, RADIAL BALL ______7	/■ jRT Ml \
    .5512 X 1.3780 X .500	,,7^777777^	(_
    COVER-AI79I73------------------------- --------------------
    PiN-^X-I
    (RIVETED)
    GEAR- BI38664A - RIGHT
    GEAR- BI38664B-LEFT
    WASHER - BEAXID SCREW - BC0X3CE
    WASHER-BE AXIL NUT - BBBXIC
    SECTION A-A ON D435O8
    Figure 9.—Indicator assembly, mirror drive.
    in actual use both arms could be either optical or mechanical, depending entirely on the choice of stations.
    (&) Carefully place the mirror assembly in position at the optical station as shown in figure 7.
    (c) Depress the latch (7, fig. 11) and insert the pivot on the mechanical arm coupler (5) into its socket in the proper index box.
    (2) Tracking.— {a) Depress the lever (8) and move the index box (6) along the azimuth circle (16) until the azimuth reported
    ’ .	14
    TM 9-2681
    4
    PLOTTING BOARDS M3 AND M4
    from the observation station is indicated on the azimuth circle and subscale (21).
    (Z>) Turn the azimuth reported from the other observation station on the indicator (10, fig. 13) connected to the optical station in use.
    (c) Position the optical arm approximately by moving the arm until the reflection in the mirror of the white line engraved on the arm appears to be a continuation of the actual line. Complete the adjustment by moving the arm until the black reticle line appears
    WORM-BI38663A-RIGHT
    WORM-BI38663B-LEFT
    BEARING, RADIAL BALL .4331 XI2598 X.500
    PLUG-AI79I7I
    SCREW - BCUXIEE
    SECTION B-B
    ON D435O8
    RETAINER - AI79I70
    FR A 1142
    Figuiie 9.—Indicator assembly, mirror drive—Continued.
    to be superimposed upon the reflected image of the broad white line in front of the telescope lens.
    (