Crusader is correct... it is the "keeper" from a mid-1800s-to-early-1900s civilian sash-buckle, made of stamped sheetbrass. At just 1.5-inches wide, it is too narrow (and also too "light-duty") to be from a military buckle. Note the EXTREMELY thin rim around the hole for the buckle's tongue-hook... if you put any weight on that belt, such as a sword or cartridge-box or pistol, the hole in the keeper would break. It can only be a sash-buckle's keeper.