I think Mike's assessment is spot on... a very typical NE type musket/fowler, more than likely dating from around 1795-1800 than the Revolution but without examining it closely its really impossible to tell... and may be impossible even with close examination.
I am extremely skeptical of the marking. While it is true that the surcharge was instituted during the Revolution, most muskets were not marked until well after the war when they were in arsenal storage and all (or nearly all) were French pattern military arms suitable for issue to the army. Many had been assembled from assorted parts, especially at the Philadelphia Depot, but virtually all were of the conventional "Charleville" pattern. I do not contend there isn't the odd exception but I'd like to see an iron-clad provenance (like having been in a known collection before the turn of the century) for nearly anything else.
I suspect that 5 out of 6 (or more) surcharges are fakes...
Years ago a good friend, who started collecting in New England in the 30s, told me he'd looked for close to 20 years before seeing his first surcharged musket and it was a conglomeration of French parts. Then, beginning in the early 60s, surcharged muskets started popping up all over the place. The bicentennial engendered a real surge in their "discovery." This was the easiest way for a dealer or collector to give any old gun a "Revolutionary" provenance and is sometimes (as I believe in this case) applied to guns it is completely inappropriate for.
