I **think** it is a brass sideplate/escutcheon-plate from a Colonial Era to early-1800s flintlock pistol or musket/rifle. I know civil war era Percussion gunparts fairly well, but not not the Flintlock period firearms your find is from. So you'll have to wait a bit for one of this forum's Flintlock experts to give you a more specific ID for your sideplate/escutcheon-plate. The photo below shows a very basic version of the more-elaborate sideplate you found.
CannonBallGuy posted side plate photos on the British Brown Bess. The side plate on a Kentucky Long Rifle can be very plain, or very ornate, they are also found on pistols, muskets and rifles, and are not there for looks. Usually there are two screws that pass though the plate and the stock, and screw into the lock to hold it in place. However there are also some firearms that only have one screw or bolt hole in them. It's more common for there to be two screw holes, but there are also some with three. Here are a couple of pictures of side plates on long rifles.
Google Kentucky Rifle side plates, and you will see the tremendous variety there is out there.
The "Like" button is there. An extra-large photo (or something else?) makes the post wider than a computer monitor's screen. Scroll right to reach the Like button. I did.