Lots of nice photos--a cheesecake gallery of Spencer details.
This is a Burnside-manufactured Model 1865. The Burnside-made carbines had "Model 1865" stamped on the top of the receiver perpendicular to the barrel. The 1865 model carbines produced by the Spencer company had "M 1865" stamped on the barrel rather than the receiver, and the stamping was parallel to the barrel. The other details of your carbine (beveled receiver top, ribbing on the magazine tube cap, beveled hammer nose) all appear consistent with a Burnside carbine, as does the Stabler cutoff (the little half-wingnut in front of the trigger), although some Burnsides do not have the cutoff.
Burnside production started near the very end of the Civil War and I believe it is generally thought that they were not actually fielded before armed hostilities ended. They were, however, quite frequently used during the post-war westward expansion by the military, Indian scouts, and civilians.
Thanks for posting the photos--it's always fun to see the details of another Spencer.
--DJ