First-second century AD. 2 ½” diameter, bronze, with broadly rounded section edge. The back with four loops for fastening. Known as concealed-loop disc phalerae
. See https://www.forumancientcoins.com/numiswiki/view.asp?key=Horse%20harness
, first three images for virtually identical examples in inferior condition to this. Used to secure the straps of chariot horses.