James, you have a very, very early M1860 Spencer Carbine if the assumed beginning serial number range according to Flayderman's Guide to Antique Arms (8th) edition is correct. I have a Spencer less than 100 serial numbers above your's that is National Archive records identified to a cavalry trooper in the 3rd Michigan.
I suggest you post any questions about the gun on the antiqueguns.com 'Ask the Experts' page for Pre-1899 Firearms. You'll get a lot of help. Good luck.