It is a "Union Loop" horseharness buckle. We date them as being post civil war, because they show up in late-1800s/early-1900s horseharness/tack/saddlery mailorder catalogs. For example, the J.M. Eilers & Co. Saddlery Hardware catalog, page 107, here:
https://archive.org/details/catalogueno200jmei
So far as I currently know of, nobody has reported seeing a Union Loop buckle in civil war era catalogs.