I looked the passage up over lunch hoping to find something helpful to figuring the mystery out. No such luck. He does indeed say "United States muskets, with bayonets, new from the factory."
If his year is correct, I can only assume he is trying to indicate they are American made muskets rather than imported guns. You see that in fur trade ledgers sometimes where they list "U.S./American" vs "English" rifles and fusils. Various states had contracts with gunmakers to produce arms for their militias. Depending on the state militia laws it was generally the individual's and/or local community/state's responsibility to arm their militiamen. It was not the federal government. Whitney, for example, had contracts with New York, Massachusetts, Connecticut, and South Carolina. The Commonwealth of Pennsylvania had contracts with various gunmakers both in Pennsylvania and England, and Virginia had muskets from the Virginia Manufactory. Standardized muskets all seem to start after the introduction of the 1795 at Springfield, but there are loads of "New England Militia Muskets" (usually basic .69 caliber musket with pinned fastened barrels) posted online, some might be from 1792 or earlier, but they are usually listed as 1800-1830s.