All that I have to add to this discussing at this point is that every one of these bolts which I have removed for service on several different 3100s have had the same 7/16" fine thread bolts with the hole for the cotter pin (next to impossible to find a non stock and suitable replacement), and a thick flat washer. Once the nut was torqued down to specs, the hole in the bolt was too far out for it to engage with the gaps in the end of the nut. (If you squint, you can see this in Tim's pic.)
If the hole had been located so as to engage with one of the 6 gaps in the end of the nut, the odds of the hole ending up lining up with one of those gaps would be less than 50/50.
I would not attempt to over torque fine threads to reach the next gap, nor would I back that nut off one little bit to make a cotter pin to line up with a gap. (A moot point, given that the holes are not located so as to engage with the nuts.)
My theory on the design is that the cotter pins are there to prevent catastrophic failure should the LOCK nuts (yes, I am standing by my position that these are lock nuts) loosen up while driving.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission