Hi Kevin,

I used to own a 78 version of the Mariner and believe what you are looking
for is the hinge for the flybridge pullout seats, at the seat joint. I know
I reused my hinges when I re-upholstered. They were wider (3 1/2") aluminum
and had some corrosion, but they are hidden by a fabric cover.

For a source, I would try to search the internet for back to back boat
seats, then contact each manufacturer and give them the sizes of the flanges
on the hinge you are looking for. Also, while out in CA, I stopped in
Minney's yacht supply and they had a lot of older loose hardware. If you
sent them a picture and dimensions, they might have something buried on the
shelves. Most of the older Mariners I have seen have had the seats removed
after they rotted out.

Good luck in you search,
Brian