Some sort of tether(?) ring at Fort Taber, an old military fort in New Bedford, Massachusetts. I am not sure of their use, but they are about 8-10″ round and solid. The weight of them being bumped against the stone they are attached to has worn away at it, leaving a ring-shaped channel that looks as though it were chiseled out of the stone. I don’t know as much about this fort as I probably should, and judging by the amount of vandalism in some spots, neither do a lot of other people.