Your find is a suspenders-strap length adjustment buckle, from the late-1800s into the early-1900s. The two "holed" flanges on each end of the top of your buckle held a (missing) "sawtoothed" crossbar, whose teeth gripped the strap firmly enough to maintain the desired length for the strap. The bottom of your buckle had a (missing) wide upturned "tongue" flange, which held a metal ring which was attached to the end of the suspenders' lower strap. See the photos below. The wide tongue-flange on suspenders buckles dates from before the civil war, but the earliest Patent-date I've seen for a suspenders-buckle which had a sawtoothed crossbar is 1868.
The one in the first photo below, which (like yours) has two "holed" flanges to hold the (missing) sawtoothed crossbar, and a wide tongue-flange (like yours) is marked "Pat. March 7, 1871."
