It is certainly plausible that it was assembled from parts for use in the Revolution, but I can't say it looks much like any of the militia muskets with mixed parts I've seen. Most that I have looked at in recent years had a mix of Brown Bess and Charleville musket parts, occasionally commercial hardware merchant locks, and sometimes American stocks. They have usually looked much like a standard military musket at a glance. Obviously this one has had the stock cut down and other additional modifications. It could simply be a salvaged surplus gun used for any number of purposes, maybe even use in the fur trade.