How large is it? Can you read the writing on it? Is it silver or nickle silver? Just off hand I don't think it's really old, as in more that 50 or so years, but that's just a guess on my part.
It is a shade over 52.1 mm (a shade over 2 inches) long by 42.86mm wide (roughly 1 11/16 inches) and is marked STERLING om the back. That is all I can make out under the loop. Ghosts of other lettering is there, but can't make out what it says.
That buckle's belt-attachment form is what I call a "bar-&-nearly-vertical-stud." The earliest example I know of it is on an early-1960s (Civil War Centennial) reproduction CSA belt buckle which is marked "Pat[ent] Applied For"... which indicates this belt-attachment form was invented and Patented in the early-1960s. See the photo below.