It is not a Brown Bess; there are too many style differences for it to be any pattern or even a copy. One important point would be the caliber; the British muskets and colonial copies were .75; French muskets and American ones made after independence were .69.
It is obviously a conversion to percussion, though when it was converted it is hard to tell; it could have been as late as the American Civil War.
AFAIK, all the Springfield muskets and contract copies were banded; that one appears to be pinned, indicating a state or private purchase.
I have the feeling that I have seen that sideplate pattern before, though when or where I don't recall. All of the Brown Bess sideplates except the later India and 1779 Land Pattern have "tails", extensions backward past the rear screw hole. And all were rounded brass; that one appears to be flat iron.
Jim