They look very similar from the side. The difference is in the width. The '39's used a narrow box like the '37-'38. (45-3/4" wide) The '40-'46 boxes were 48-1/2" wide. The runningboards on a '39 and a '40 are in the same basic location. This means the '40-'46 rear fenders had to be 1-3/8" narrower than the '39 rear fenders to make up the difference in box width. If you lay a '39 rear fender flat on the floor and measure from the floor (box flange) to the top of the fender, it will be approx. 13". Measuring a '40-'46 rear fender the same way will give you approx. 11-1/2". Good used '39 PU rear fenders are very hard to find since they fit the one year only.
