I think any reference to USA or CSA mounted horsemen (cavalry if you will) carrying upwards of 4 revolvers is a myth, except for possibly the Confederate guerillas in Missouri like Bloody Bill Anderson's or William Quantrill's groups, who would have used captured Union revolvers and carried them in saddle pommel holsters rather than primarily on their person. The captured Union revolvers could have ranged from 1851 Navies, 1860 Armies, 1861 Navies, or even 1848 Dragoons, and I would think those guns would have been spread out amongst the group members so as to have the men somewhat equally armed. I highly doubt spare loaded cylinders were carried for such revolvers as it would be non-conducive for hit-and-run raids like the aforementioned groups used. Retreat to fight another day and then reload the revolvers for the next foray.
The act of even attaching a shoulder stock to a Navy or Army pistol takes a bit of time, and then one would have to figure out how to carry it. IMO, a very poor idea but Colt was trying to muscle into the carbine market with the arms he produced, primarily revolvers.
I cannot envision a horseman using a Colt revolver with a shoulder stock on horseback. It would have taken two hands to properly aim the "carbine" for 6 shots leaving the horse reins free, so it may have been something that a dismounted cavalryman could have used.
With carbines like the Sharps, Spencer, et al, a dismounted cavalryman with a supply of ammunition could reload the gun until the ammunition ran out and they would have been much better than a 6-shot pistol carbine with no possibility of a reload.
Just my $.02 worth.
Regards,
Jim