It isn't a buckle... it is what was called a "buckle shield," used on horse-harnesses. The version you found was invented and patented by S. Mosman in 1879. See the Patent-marked one in the photo below ("PAT. DEC 16, 1879"). The patent expired after 17 years, and therefore the Patent-marking does not appear on ones made after the expiration date.
I'll also include a scan of the original 1879 US Patent document's official "Diagram" and "Description."
the buckle (or better, buckleshield as Cannonballguy pointed out) is stamped and therefore factory made. I would not date it older than second half of the 19th century.