Dave, to me it looks to be a model 1842 Springfield .69 cal smooth bore as I don’t see any signs of a rear sight. Although by 1864 most yankee units had switched to rifled muskets, there were some units in the iron brigade that preferred the smoothbores because of the buck and ball ammunition. Confederates used whatever was available. It could also be breach conversion 1816 to I guess, hard to tell from your pictures but it should clean up nicely. Great job!! If you find the hardware, the will make life easier to identify. I’ll post pictures of a few civil war muskets from my collection. The pictures are both of m1816 percussion conversions done before the war, the top musket is a “cone” conversion and the other is a breach conversion. I don’t think yours is a conversion but it could be as a lot of different arsenals did the job in different styles. I’d put my money on a ‘42,