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I assume you're referring to the four "carriage" bolts next to the floorboard. Mick Isbell's hardware reference shows A-20922 as a 3/8-24 x 2-1/4 carriage bolt, which is a contradiction, since carriage bolts, like step or elevator bolts, are coarse thread by definition. From the fuzzy pictures I've managed to find of these bolts, the head appears to be wider and flatter than a carriage bolt, like a step bolt. On my 30 standard coupe I found the PO had installed standard 3/8-16 x 2-1/2 carriage bolts in the round holes with the heads sticking up above the square shoulders - also unnoticeable under the carpet.
Since several body bolts were completely missing I ordered a set from one of the vendors only to find they provided the same standard carriage bolts, but at least of the correct length. I couldn't find step bolts anywhere in either fine thread or 2-1/4 length, so I decided to simply grind off the square shoulders of the ones I had received and install them as is. There is sufficient friction on the heads to prevent them from spinning.
If anyone has these original bolts, it would be good to know their actual dimensions.
On a side note, the flat washers in the kit have no use that I can find; you can't start a nut with both flat and lock washers in place.