Your find appears to be a pants-waist strap adjuster buckle. It was also used as a vest-waist adjuster. See the photos below. That specific type has been found marked with dates from the 1850s & 60s... so we do know for certain they go back that far. Of course they can be more recent. In fact, they are still being manufactured today. But yours is thin stamped-brass, with raised-lettering marking, and made in France, so I'd say yours dates from the 1850s-70s. The one in the closeup photo below has indented lettering (on the crossbar, saying "Make"), so it dates a bit later than ones which have ONLY raised lettering.