Next, an deck inspection port - sometimes I think these are used for fuel tank filler access:

This particularly filthy example is on B.A.S.P., a 45' Watson from 1924, and a much cleaner example:

as the fuel filler access on Mary Gabriel, a Rother.
Lift the semi-circular ring, unscrew 3 or 4 turns, and lift off. The unscrewed bit
should be held on with a bit of chain, so you can't drop it into the briny.