More info for you, from the book "American Military Belt Plates" by O'Donnell & Campbell.
Your plate is a waist-belt plate, Regulation 1874 Pattern, Army Officer. Made of stamped sheetbrass. Circa 1890s through early-1900s. The book says "These were a staple of late-19th and early-20th-Century military supply catalogs. Primarily nonregulation, large quantities were sold to the civilian/quasi-military market, and perhaps to National Guard units as well."
Your plate's belt-attachment hook and bar were soldered onto its back. If you look closely, you can see the grey-ish solder marks where the hook and bar used to be.
Quick-ID clue, for relic-diggers and collectors:
The Model-1874 version differs from the civil war era version by not having a rectangular belt-slot on the plate's left end.