Fyrffytr1 is correct. More specifically... your find is a Mosman's Patent 1879 "buckle-shield." The life of a standard US Patent is 17 years, so all of these buckle-shields made during the 17 years between 1879 and 1896 were marked "Pat. Dec 16, 1879" -- and ones made after the Patent expired are unmarked. See Mr. Mosman's original US Patent application & diagram, attached below, and a scan from an 1895 Montgomery-Ward horsegear catalog selling them (which mis-spelled Mosman's name), and a photo showing a Patent-marked one.