It looks similar to another item in this section that was posted later today. That was I.d.'d as a guide for a toilet valve. Of course, if yours is iron, it's not likely to be the same thing as it should be brass.
A couple of more pictures may help, notably where your fingers are!
It could be a hanger for cast iron pipe, a handrail bracket for metal handrail, etc., whatever it is, a piece of pipe of some type went through the "hoop" IMHO.
I believe it may be part of a farm gate hinge. The bolt goes through the wooden fence pole and bolts on the other side to hold the hinge tight. The gate would be made of pipe which would be in the loop....