It is a US Army Model-1826 shoulder-belt plate for NCO (Non-Commissioned Officer) ranks, such as Master Sargeant. By very closely examining tiny details in your plate's emblem (such as the hollow-ended olive branch), I can tell you with certainty that your find was manufactured sometime between 1855 and 1865 by John A. Pittman, who was a major supplier of belt-buckles and plates to the US Army during that era. For verification of what I'm telling you, see info and photo of Plate #458 in the book "American Military Belt Plates" by Michael J. O'Donnell and J. Duncan Campbell. Your shoulder-belt plate's three attachment-hooks were made of "doubled iron wire," which has almost entirely rusted off the back of your excavated plate.