it is a Beckson 6" pop-out plate. (its a 6" opening so they call it 6". Do not get 8" it will be wrong) They come in a screw in type also. They come in white, beige, grey, black and clear.
You can get cheaper brands also.
For decks, the pop-out type are best. They don't have an indented finger space to "puddle". For motor wells and brackets, get the screw in type so they can't pop out and sink your boat.
They will say "do not silicone" but everyone uses silicone....