At first glace it looks like one of the many varieties of suspenders-buckles. But comparing its width with your finger, it is too narrow to fit a suspenders-strap. It is a men's garter-strap buckle. It closely resembles the version at the upper right end of the strap on "the Boston Garter" shown in the early-1900s advertisement below. The "tongue-hook" on your buckle held the cord, as shown in the ad's drawing.
The Boston Garter's manufacturer was the George Frost Company. It has been in business under that name since about 1880, and is still in operation today. That being said, the "cloth-clasper" on the Boston Garter (see photo below) was patented in 1895... so your garter-strap buckle dates sometime from that year on into the early-1900s.