Q. Do any of the wear iron bolts go through the crossmembers? All of them? None of them?
Not sure, there could be some wear iron bolts that line up with the crossmembers.
My 54 has a factory marine plywood floor and the only holes in the crossmembers are at the frame mounting points.
The wear iron bolts are short.

Q.Am I correct to assume these just sit on the frame rails?
I not sure about how it is for a 1937 panel.
If they make a 1937 factory assembly manual it would be good investment.