I was told that the 1939 rear fenders were one year only, the 39 has a 45 3/4 inside bed width and the 1940 to 1946 trucks were 48 1/2 and that the extra width was in the rear fenders. I picked up some 1938 parts last night and suspected the rear fenders were actually 1940-46 because they did not have the style line around the wheel opening that 1937-38 fenders have. but when he showed me the bed (he used the bed on his truck), it was really narrow, and had a very small upper roll diameter (maybe 1 1/2?) so I thought that it might be 1939.
I put the rear fender next to the 41 rear fender I had in my parts and sure enough, its wider. the 39 is the lighter colored fender, the 41 the black.
IMG_1429 by
Joe Doh, on Flickr
IMG_1430 by
Joe Doh, on Flickr
IMG_1431 by
Joe Doh, on Flickr
IMG_1432 by
Joe Doh, on Flickr
it looks like the extra is split over the crease, appx 5" from crease to mounting for the 39 and appx 4 1/4 on the 41.
thought it was interesting, now I have seen it. the pictures may not show it well but the difference is pretty obvious in person.