Small? A couple thousand of them. When the British left the enemy in possession of the field
The original question was whether or not there was any documentation of military muskets being trade items. Battlefield finds are a totally different question.
First, more on scale of trade fusil availability. Literally thousands of trade fusils were imported each year. In later years when American manufacturers were finally able to compete with the English, Henry Leman alone had a contract for a single year to produce 6500 NW guns and another contract the next year for nearly that.
I am not a serious student of the F&I or Rev war eras, but I find it hard to believe that even in Braddock's defeat and retreat 'a couple thousand' functional muskets would be left laying on the battlefield. Especially since most of the troops present were militia who generally brought their own arms. Even if they were a 1000 muskets dropped here and there it would be a blip on the larger scale of fusil availability. Others may be better speculators than I on that issue, but muskets would still not be trade items even if it were the case. My interest is mostly in trade arms and mostly a later time period. I welcome speculation or documentation from others who are focused on these earlier eras.
I did find a bit more documentation on 'military type' muskets in the trade. In 1839, Henry Deringer had a Government contract for 900 'guns for Indians', all were to be of 32 gauge (~.54 smoothbore, much smaller than standard military muskets). Half of these were to have barrels of 2'10" and the rest were to be 3'10". At least some of the latter apparently used musket locks with brass pans from 1816 muskets. Of the shorter models, the only known example has a cheap British export pistol lock. The contract also specified that the short models would have sling swivels. This contract was issued on January 10, 1839 and Deringer delivered all 900 on May 27 of that same year. There are no records of similar contracts going to Deringer. Those in subsequent years specified standard NW guns. For documentation, see 'Henry Deringer's Guns for Indians' in the Museum of the Fur Trade Quarterly, 1967, volume 3, #4, pages 5-9.
Sean