My notes suggest there are two, near the top of each seat back. These are 1-inch long, #8-18 threads, Pan Head Phillips drive. Silver in color, likely zinc and not chrome or stainless. But any of those finishes would work.
Are all the screws the same length? I think there are 6 of them all together and then the seat backs hook into that big lower bolt on the
bottom of the seat on each side.
The ones for the side trim and other seat parts are different. You only mentioned the backs. Maybe images of the parts you want screws to help clarify?
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I have my original ones which i'm going to clean up, scuff up and paint to make them look nicer. They have alot of scratches on them. When I'm done
they have to be screwed onto the back of both seats as in 1970 it wasn't a solid seat. More like 2 parts. The seats and the seat backs.