on my '40 the block my fenders bolts run through are solid. In both my parts books they are not solid. My 1946 parts manual says 9N-4012-B2 is used from '39 to '42. Any idea when they changed?

I was wondering the same thing. I have some trumpets from 9N's that are solid. The oldest s/n in the group is 9N47611 (1940?). I also have 9N295654 (1947) that has the open block axles. So I can authoritatively say the changed happened sometime between 1940 and 1947. How's that for going out on a limb?

My '40 is 9N3598I. So that would put it in April 1940? Just a guess here ofcourse.

I am pretty sure they changed in 1941, do not know of serial though.