I know it's not a hog ring, I've raised hogs and cattle that also have rings, sometimes. In fact there is a bag of them out in the barn along with the installation tool. Probably not a door knocker because I've found three, all on farms and I've never seen a door knocker on a farmhouse door, that's more of a city thing (LOL) , not to say that it's impossible. Besides, they are usually made of brass or a non rusting metal. Also I've got several block and tackle sets made in different styles, here on my farm, none of them have anything that resembles this thing. Maybe it could be a chain guide as suggested...
Richard Ray