The serial number could affect the value in this case, so I'd like to be dealing with a full deck. FWIW, I have no interest in buying the gun, if it were for sale (which I'd advise against, strongly) so, I'm not going to "low-ball" you.

If you have a cabinet (studio) photo, then the arm very likely is a prop. Do you know which unit he served in? Cavalry had carbines such as yours, the infantry had the full-length rifles with long wood and 30" barrels.

What you have described is a not uncommon scenario. Guys would sometimes purchase an arm similar to that which they carried. And, in some cases, during that era, soldiers were even allowed to purchase their weapon(s) at time of discharge.

So, it COULD have been his arm, BUT he would have had to have been in a cavalry unit, AND, he "played" with it, post-service, since that is not an official configuration.