TheCannonballGuy said:Fano169, while doing research on another type of belt-buckle today, I came across a couple of VERY-nearly exact matches for what you dug. They are a 20th-Century horsegear Surcingle 2-piece buckle. Compare the photos below with yours.
Your find is half of a brass 2-piece buckle for a leather or fabric belt or strap. The version you found dates from the late-1800s into the 20th-Century. Here's a photo of a similar (but not exactly the same) version on a Model-1917 US Army "garrison belt."
Perhaps somebody here can provide an exact identification for the version you found.