There are a few that I am aware of:
1946 has 2 more bolts holding on each fender, so there are two more fender bolt holes in the bedsides.
1941 has a different fender bracket which requires an additional bolt hole on each bed side. There is also an L-bracket welded onto each side of the second to back bed cross member to reinforce the connection of this fender bracket to the bed.
Some early production 1941s have no cross bolts running through the rear bed mount wooden blocks so the two holes provided for the purpose on later bed sides are not present.
1946 bed sides have a small plate reinforcing the slot for the tailgate chain.
There was also a change to the bracket to which the spare tire J-hook attaches, but I believe this happened during 1941. The later version required a hole through the rear cross member and the earlier did not.
