It sounds like you have a Spencer Carbine that may have been modified. The Spencer saw some changes during the war and saw a lot more during the Indian Wars of the 1870's. They made a 39.25" length Spencer Carbine with a 22" barrel length. Check your barrel length and see if it is 22". If the barrel length is about 21", then you may have one of the 39.25" carbines that has had an inch cut off due to damage or for some other reason. Does it have more than one barrel band, if it has more than one barrel band, it could be the longer Spencer Rifle that has had several inches cut off. In any case the front site would had to have been moved. After the civil war many of these Spencer Rifle/Carbines were customized. They call it sporterizing and it was done by everyone from civilians to the American Indians. The production of the Spencer was stopped about 1869, but various types of its ammunition was made into the 1920's. And today some specialty companies are still making ammunition for the Spencers.