Is it Civil War? NO

This is a Model 1865 .50 caliber Spencer Carbine with a Stabler magazine cut-off switch The Stabler cut-off was patented on March 14, 1865 and allowed the carbine to convert from single shot loaded at the breech to magazine fire loading only via the magazine. The Stabler cut-off switch shown in the two bottom images was not incorporated on any carbines prior to those shipped on June 16, 1865. A total of 12,502 carbines are manufactured with the cut-off. Thus, this weapon did not see any service during the Civil War.

Reference: Marcot, Spencer Repeating Firearms p. 80