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Solved...

I found a few photos of two other SS boats that happened to have the seat hatch open. I saw how the seats are supported and that they indeed did have hinges.

Today, I fabricated the upper and lower seat "cleats" for both sides. The stainless screws that go into the upper cleat were still attache to the boat, but I guess the wood pieces rotted away.

While I was doing these repairs, I decided not to repair the screw holes in the hinges (seat and backrest). Instead, the are not through bolted. Problem solved forever with this quick fix.