I did some research and have decided that this is an English Long Land Musket, circa 1768. The missing escutcheon plate from between the comb and barrel tang is a strong hint. The symbols on the butt tang appear to be 53 over H over a number. The 53 is for the 53rd Regt., part of Burgoyne's invasion force in 1777. The H stands for the 8th company under Capt. Baird which was captured at Fort Ticonderoga by Col. John Brown during his raid of 09/08/1777. Below the H should be a rack number. On the lock plate behind the hammer is the maker's name, F LORD, and below that should appear the year it was made.
This info and some pictures appear on page 61 of Battle Weapons of the American Revolution by George C. Neumann.
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