Kent,
These are, it is very likely, American-made muskets, many of them having been rounded up from non-associators, owned personally by the soldiers, or "public" armaments that survived from the French and Indian War. Pennsylvania conducts a furious effort in 1775 to collect muskets. There's a fantastic document among these same papers (Papers of Pennsylvania's Revolutionary Governments: 54 microfilm rolls) that identifies the persons from whom muskets were taken (in and around Philadelphia). Many, too, were newly produced by Pennsylvania's gunsmiths after the revolutionary provincial government compelled them, county by county, to produce quotas of stands of arms.
To me, this sort of detail strengthens the argument that these numbers were put on arms (muskets, in this case, but also rifles used by soldiers in companies) long after production.
Scott