Looks like a drain plug
to me as well. Unusual, but could be intended - boat stored ashore for long periods and known to fill with water
(melting snow?) and an answer to frozen water
in the bilges.........or could simply be a bodge to fill a vacant thru hull
using whatever was to hand - especially if no access from the inside to make a proper fix to blank off easy.
An outside chance that could be the drain for an inbuilt water (or fuel
?) tank, but the design seems odd (nothing to snag would make more sense) - FWIW, I have a couple of drains (flush design) in my bilge
keels which are also water tanks! (very unusual is that!).