In the Union Army it was the norm for a regiment to be armed with all the same type weapon. Chances are that the owner of that carbine was in the Cav. although I believe there were some infantry units armed with Spencers. When he went to the hospital his weapon probably stayed in the care of the company quartermaster. When he came back he may have gotten the same weapon back or another Spencer. The Spencers had serial numbers but I have never seen any records where the serial numbers were written down for individual issue.