I'm unsure and just thinking of ideas. This picture is an engine block drain plug off an old Farmall tractor engine. Removing this plug would allow you to drain out the water jackets of the engine block. In the old days draining the water coolant out for an engine that's gonna be parked all winter would be routine in some cases. Or maybe to just change the coolant to a fresh mixture of water and antifreeze. Yours looks smaller and to me it doesn't look like brass, so maybe it was a drain plug for an old water cooled hit & miss engine 🤔