Interesting item. Well worn at the attachment point. The pin was make by a blacksmith, who heated the iron, wrapped it around the ring and then hammer-welded it together. Must have been hard to not over-heat the brass. The pin may have been set in concrete or mortar or the protrusions on it may be what is left of heavy screw threads which might have been screwed into a pre-drilled hole in wood. Fun to speculate. Must have been a strong metal detector signal, round and brass!