Tethering ring or Cast iron heavy circular door knocker ,although it could also be used  in the house as a towel holder etc.  Because it was used as a tethering ring, it does NOT have a plate if used as a door knock, it will go straight on to the door, . Heavy, weighs 982g, diameter 4.3/4"inches, base plate diameter just over 2.1/2" inches. Postage by Royal Mail 1st class small parcel signed for.