The Master Parts Catalog says 1939-1946 fenders are interchangeable. There is one small possible difference for the '39 version and maybe the '40? The fender brace is located about 1/3 of the way up from the bottom rear. The later version has the fender brace at top center. I am unaware of any other dimension differences. It seems to me if there were, the parts numbers would be different. My guess is, if some have or have seen different widths of the fender(s) these were intended for use on 1 ton and larger trucks. Another possibility is that in '46 after war production ramped up quickly and as I understand it some of those models had parts somewhat cobbled together to get units to dealers ASAP.