"If someone has a copy of the AAR Recommended Practice Manual, C-42,
then we can get the official word on the subject. :-)"
From the 1954 AAR Manual of Standard and Recommended Practice. This revision includes the modifications made with regard to the position of placards (circa 1954).
Further information is contained within Interchange Rules 36 and 49 as to the requirements (from owners) for cars offered in interchange. I will try to post that information within the next couple of days.
HOUSE CAR, PLACARD AND ROUTING CARD BOARDS FOR
Adopted, 1914; Revised, 1948, 1952, 1953
The space available for placards should not be less than 16 by 24 in. on each end and each side of car. Steel house cars should be provided with placard boards made of soft wood not less than 16 by 24 in. and not less than 23/32 in. thick. The vertical edges should be reinforced with metal protection pieces fastened to the board with not less than six bolts, three through each, and at least four of these bolts should be used to fasten the placard board to the door, car end, or placard board brackets when used. The boards may be made of more than one piece, and should then be tongued and grooved, ship lap or notched edge. The distance from the floor line of car to bottom of board should not exceed 2 ft. 6 in. for end board and 22 in. for side boards. These will be located on each end and on one door on each side.
On steel house cars routing boards with space of not less than 51/2 by 9 in. available for tacking cards will be placed on one door on each side to left of placard board with lower edge in line with lower edge of placard board and so as to not interfere with lifting handle of door. On steel refrigerator cars, routing card boards shall be located on each side directly under placard boards.
On other than house cars, cardboard must be located on each side of car near bottom at left hand end, facing side of car, or on outer end of end sill. On cars equipped with center sills only, cardboards must be located near center of car attached to outer end of running board support, or attached to outer end of body bolster.