I've sailed and re-rigged small full scale yacht
s and catamaran
s all my life and never seen a fitting like that. Perhaps it was put there as a halyard or stay 'tidy' for storage purposes? looks more like an upside down boat cover
tie down clip or part of a latch. Perhaps try the door and lock section at your local hardware super store. Found this on google. As the other guys have said though, probably hand made. Usually, no rigging fittings
s have square edges unless they are part of a fitting.