Here's the section of the pertinent Army manual for painting an ambulance in the late 1960's:
16 - Ambulance Marking - Military Design Vehicle.
a) Truck, Ambulance, 1/4 Ton, 4x4, M170 (Series).
(1) An 18 inch red cross on a 20 inch square white field will be marked on each
canvas side curtain directly to the rear of the canvas side doors.
(2) An 18 inch red cross on a 20 inch square white field will be marked on the rear
of the vehicle. The upper half of this marking will be applied on the canvas rear
doors and the lower half on the tail gate, centered with the centerline of the
vehicle, in order that the full marking is shown when the rear doors are closed.
(3) A 45 inch red cross on a 48 inch square white field will be marked centered on
the canvas roof.
b) Truck, Ambulance, 3/4 Ton, 4x4, M43 (Series).
(1) The word "Ambulance" will be marked in 3 inch high white letters, centered
three-quarters of an inch above the windshield.
(2) A 3 inch red cross on a 4 inch square white field will be marked to each side
of and in line with the word "Ambulance" above the windshield.
(3) A 36 inch red cross on a 39 inch square white field will be marked on each
side of the body below the roof joint and centered above the rear wheels.
(4) A 33 inch red cross on a 36 inch square white field will be marked on the rear
door panels directly below the windows and centered on the vertical centerline of
the vehicle.
(5) A 60 inch red cross on a 64 inch square white field will be marked on the roof
14 inches from the rear of the vehicle (figure 20).
Figure 20: Ambulance marking for military design vehicles
c) Other Vehicles. Vehicles other than the ones enumerated which are intended for such
usage and which are affected by international agreement concerning such vehicles will be
marked in a similar manner.
d) National Symbol. The National Symbol will not be applied to Medical Service
vehicles.
e) Sizes. The' sizes of the red crosses specified are measured between the ends of
opposite limbs. Each limb of the cross will be a square having sides equal to one-third
the size of the cross.
f) The Army Medical Service Insignia. The Army Medical Service insignia consisting of
a red caduceus on a circular white field will be applied to each side of the ambulance.
Caduceus to be Type 1, MS 16686.
(1) The circular white field will be 91/2 inches in diameter.
(2) The red caduceus symbol will be 6 inches high.
(3) The letter U will be centered to the left of the caduceus symbol; the letter S
will be centered to the right. These letters will be red, 1 1/2 inches high.
g) M170 Series Ambulance. The insignia will be applied each side of the vehicle body to
the rear of the rear wheel.
h) M43 Series Ambulance. The insignia will be applied on each side of the vehicle body
centered horizontally forward of the red cross emblem. The top of the white circle will be
level with the top of the horizontal red cross arm. The legend "Army Medical Service"
will be applied centered 31/2 inches below the caduceus insignia, in 11/2 inch high white
letters.
i) Ambulance Color Markings. The following colors will be used for ambulance
markings, as applicable:
(1) Gloss red, Color Chip No. 11136, and Gloss white, Color Chip No. 17875
on vehicles painted gloss or semigloss olive drab.
(2) Lusterless red, Color Chip No. 31136 and lusterless white, Color Chip No.
37875 on vehicles painted lusterless olive drab.
