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Familiar with the 68 through 70 C3's. Don't recollect ever seeing anything like these.
Thread size? Are they stainless? If so. they may nevertheless be mildly magnetic. Test they're ferromagnetic by comparing with a known steel bolt. The steel bolt will be much more "sticky." I don't think there's a stainless fastener anywhere on a C3 that I've worked on.
They obviously are used as a pivot point, being a shoulder bolt, so, depending on the size, they could only be used in a few places.
Seat back hinge, seat belt, wiper door, headlight door, etc. Might not be oem, which would make the search harder.
It's hard to tell the drive head... Phillips or Torx or Allen...
My '67 has no such fasteners that I can recall. If Corvette, I think these are much later in the C3 or C4 era.