Your find is definitely a (civilian) bridle rosette. Stamped sheetbrass front with solder-filled back. (Yes, it is solder, not lead... most experienced diggers know pure lead doesn't corrode like what we see in your photo -- but a lead-alloy (lead plus zinc or lead plus tin) does corrode like that. The wire embedded in its back for attachment to the bridle's leather strap got pulled loose... which is probably how your rosette got lost. This particular construction of rosette (solder-filled back) dates mostly from the late-1700s through the mid-1800s.