My good friend Creskol is correct about horse-harness relics, as usual. MikeRo, here's some more-specific ID information. Your find is a Mosman's Patent buckle-shield a.k.a. "buckle cover." It was invented and patented David Mosman of New Britain CT in 1879, the date Creskol said.
The images below show:
Mosman's US Patent application describing his invention,
the diagram of it from his patent-application,
a specimen which is patent-marked with the issue date (Dec. 16, 1879), and
a scan from the 1895 Montgomery-Ward mail-order catalog selling this version of buckle-shield (which mis-spelled Mosman's name).
Under US Patent law, the Patented device was patent-marked during the patent's 17-year lifetime. After the patent expired, anybody could manufacture them without having to pay the inventor a royalty (or license-to-manufacture) fee, and they were not marked. If your Mosman's buckle-shield has no patent-marking, it was made after the patent expired (1896).
