The size of the letters in comparison to each other and which letter is more "behind" the others indicates the emblem says RBS. Your buckle is not seen anywhere in the book "American Military Belt Plates" -- which shows national army & navy and state militia plates. So, I suspect the B in RBS stands for Boys and the S stands for School.
Sorry to have to say, it is apparently not a civil war era buckle. Its particular version of body-style (solid-cast brass instead of stamped-sheetbrass, with square corners, a vertical back-bar instead of a slot for attaching the belt) is shown in the American Military Belt Plates book as being 1870s-and-later. Also, the peculiar lettering-style on it was popular from about 1875/80 into the very-early 1900s.