Your picture looks as if it was made of iron??It is an insulator, but I'm not sure what for. (Power lines. telegraph, etc). It would help if there was something in the picture showing size.
Iron over composite. I believe they were mainly used in trolley operations. Not sure of the advantage. Perhaps strength/ruggedness.Seems kind of counter-intuitive to have an insulator made of metal? If not an insulator, is it some kind of cleat from a dock for tying off your boat?