Civli War carbines were 22" and rebarreled in 1865 to the 56-50 cartridge and a 20" barrel.
My 1865 was made in 1864 and was probably rebarreled to the 20" 1865 model.
Hope this helps some. Good luck with your original Spencer.
Save your original rimfire breech block close the the Spencer, with a tag wired to it marked with the ser.no and info.
I take mine out and hunt with it, did some CAS with it, and have used it at cavalry reenactments. But now I like to just hunt with it as that is easier on it, and she has had a 140+ years of life already.