I’m having trouble finding this seal. Its like a flat O ring. About 3/16 wide and hi.
Thanks in advance

What are the inside and outside diameters?

It should be the same as the gasket that comes on the (stock size) oil filter.

What engine ? Maybe #8 in the attached:
https://www.marineengine.com/parts/m...100JS/1939_280

get a veneer caliper and get some better measurements of the groove on the adapter. My guess just looking at the pic is you are somewhere in the -334 to -342 range on the o-ring charts
i know this place has the square o-rings, im sure there are other places to get them as well....and it may be a custom size too.
Square O-rings

Its for a 502. Ill get some measurements

I just looked at the picture of your adapter again. Are you sure that is the correct gasket ? Its hard to tell for sure but yours almost looks like it is made for a round "O" Ring ? Does the oring fit into that groove in the adapter ?

Im not sure if it is correct. Its what came out. The grove is square shaped though, not a concave like an O ring

First thing I'd do is compare it to the gasket on your oil filter.
If its not the same, then I'd measure the OD and go to Walmart and find an oil filter with the same dimension gasket.