It's a pants-waist adjuster buckle, from the mid-1800s into the early-1900s. (But a few are still being made and used today.) See the photos below, showing a couple of them "in place" on the pants.
I've seen a few used for the same "adjuster/tightener" purpose on the back of a vest.
They can be recognized by their two-tines-on-a-bar form, and small size, and especiailly, having sharp narrow tines for piercing the cloth to get a super-firm grip.
Some French-made ones have the year-date stamped on them, such is 1857, 1858, etc. Similarly, some are marked simply "Paris."