kind of reminds me of the early type radiator caps,,they had a cork/rubber ? peice above the nut and when tightened down, swelled the cork/rubber ? out .....just a guess.........gldhntr
Looks like a plumbing piece that was used on old sinks to hold a chain & stopper. It was mounted through a hole in sink, nut was tightened underneath. Top brass piece would hold a small chain with a rubber stopper.