Confirming... it is the correct shape and size to be a flintlock musket's gunflint... and the four evenly-spaced flakes along the edge in one of the photos show it is indeed "worked," not a naturally-occurred chunk of flint. Those are the ID-clues which were given to me several decades ago after I found one (a gunflint) when digging in the Confederate trenches at Port Hudson. For anybody here who hasn't seen a gunflint in its "ready-for-action" location, see the photo below.