Keith,
A good question & yes, if on the battlefield, & if the need arose to have to replace a cylinder, you could use a cylinder from another pistol & the only difference would be the cylinder would have a different serial number than the rest of the pistol. Also, another scenario that happened was a group of soldiers cleaning their pistols, would sometimes inadvertently mix up cylinders when assembling them.
Now, if for some other reason not related to battlefield use, & you had a Colt pistol that needed the cylinder replaced, you sent it into the Colt factory. Colt being quite frugal might use parts that for some reason, were condemned by the military from being used for a military pistol (after the pistol had been assembled, stamped & test fired) & the parts were then used for whatever reason they (Colt factory) deemed fit. A condemned part wasn't necessarily defective but just didn't meet government standards. That practice continued through the Indian Wars.
Anyway, now that cylinder had a serial number already now stamped on it. But... Colt not wanting to waste anything, would use that cylinder on any pistol sent to him for cylinder replacement.
What they would do, was take that cylinder that already had the 4 numbers stamped on it, & stamped the last 4 numbers of the pistol sent in for cylinder replacement, beneath the exisitng 4 numbers.
Interestingly, I have an example in my collection.
Collectors use to be narrow minded when it came to a pistol with mixed numbers & usually those pistols with mixed numbers were considered bottom feeders.
But within the past 25 years more research has uncovered information pertaining to some mixed numbered model 1860 Colt revolvers but they have identifying mark on them that other mixed number Colts don't.
After the war, a need for a large caliber pistol was necessary to equip the cavalry out west. Since the army had a large arsenal of pistols, a large number of 60 Colt armies as well as some Remington's, were sent to the arsenal for refurbishing. All parts were disassembled, polished, & re blued with no attempt of assembling by serial number. On the Colt's, a U.S. was stamped on the trigger bow (front of T.G.). This is the only way to tell an Indian War percussion Colt with mixed numbers from any other 60 Colt army with mixed numbers.
Remington's can be more difficult to discern as they did not have the U.S. stamped on the trigger guard but some researchers feel that some regiments marked their pistols as some have been found with such marks but I think the jury is still out on that.
The military frowned on regiments marking government property. But, never say never.
It gets a little confusing at times but is utterly fascinating.
Hope this helps somewhat.