Kuger is correct, your belt buckle's form is known as a "False 2-Piece" buckle. Specifically, your buckle is a US Navy Officer buckle, Regulation 1876 Pattern. Your exact buckle is shown in the 1877 catalog of Horstmann & Brothers Co., a major dealer of militay gear in Philadelphia PA. The version in your photo continued in use up until at least World War One. Also, see buckle #1019 in the book "American Military Belt Plates" by O'Donnell & Campbell.