Bob,
I agree with your reading of the '32 chassis parts catalogue, although I note several apparent errors elsewhere in the lamp section that makes me wonder if the only way to know the difference between the two subject lamp bodies is to check the drawings at the BFRC. a/ It could be that the difference lies with the shape and size of the opening for the license plate lamp lens as the lens are evidently slightly different as noted in the '32 catalogue and pointed out by motordr. He is incorrect however about the position of the license plate lens on '32 lamps. They were only upside down compared to Model As on passenger cars. On '32 commercial vehicles and trucks, including the deluxe panel delivery which had a stainless steel tail lamp, the license plate lens is at the bottom of the lamp body.
a/ The errors I noted where for B-13408-A (they were stainless steel, not chrome plated) and for A-13445-B which suggests that the '31s had no white lens (or at a minimum they were not supplied for service, which was highly unlikely).