The small "stamped sheet-brass" dome with two attachment-tabs projecting from the edges of the back is a form of (civilian) leather ornamentation, although sometimes used on other materials such as fabric or plastic. They date from the latter-1800s through today. Those attachment-tabs on the dome's edge are the key ID-clue. Here's a photo of the back of a similar one, marked US instead of being like your "plain" one, from early-1900s US Army horse-harness.