The fact that your flat-bodied brass "hook" has only one attachment hole excludes it from being a sword-belt hook... or other kind of similar hook (such as a rifle/musket sling adjuster hook)… because those types have (at least) two attachment holes. I believe your one-hole brass hook is the "tongue hook" from a pre-civil-war Military belt buckle which had a solder-filled back. (In actuality, those buckles were filled with a solder-like alloy, not lead.) Your find is missing the single-hole crossbar which held the other two belt-attachment hooks/studs. Also, note that those other "hooks" (studs) are round, not oval puppy-paw hooks.
Your find could have come from a rare round-studs "lead-filled" oval State Seal buckle, or a very early version of US-in-an-oval buckle.