I'll have a good look around, I may still have one of the originals, although I took it apart about 10 years ago and threw out most of the really rusty bolts.
I am not familiar with any cars other than 37. On the 37, the apron bolts onto the fender perimeter (on top) and the side panels fit (nestle) in between the fender and some apron tabs. For a good clean fit, the bolts must be very low profile. Perhaps that is what drove the 1/4" size?
Note, for the apron to frame connection points, 5/16" bolts were used.