The cartridge you display isn`t a Spencer cartridge as all of those were of Rimfire design.
It is a possibility that your round is a CF round for one of the factory converted Sharps rifle/carbines but it looks to be too short OAL for that. They were converted to 52-70 and 50-70 cal CF after the N/S War. Measurements of your cartridge would be a lot of help to ID it properly. Rim dia, base dia, mouth dia, and OAL. There were many CF cartridges that look much alike during that era.