I'm 99% certain these bolts were ONLY sourced by size and length from a local ( to SAI) industrial hardware supplier. SAI did not act like FoMoCo and "spec" every part. A friend has an original SAI "bill of materials" for '65 and the next time I visit him , I will look at the call out for the bolts. "My guess" is 1/4 - 20 bolt X/X length , and nothing else.
Randy