This appears to be a faux buckle, with no actual belt loops on the sides— not a functional buckle. As such, it may simply have pinned or looped onto a sash, which would be consistent with its lightweight, die-stamped construction.
However, I should point out that the same construction is also consistent with Dg39's suggestion that it is the front shell of a buckle which had hooks, loops, etc. on a backing plate (or possibly, although he did not mention it, embedded in lead backfill).
At any rate, its design imitates that of a nonregulation Navy dress belt plate, of a type which was not required or prescribed for official dress, and which would therefore have been privately purchased. As documented in American Military Belt Plates by O'Donnell & Campbell, plates of this and similar ornate styles were in use as late as 1930. Here is an illustration of one of the functional plates, as advertised in the 1925 Bannerman's catalog: