"Artillery" length SAA
With all due respect, I believe ALL standard military procurement of the Colt SAA was in the 7-1/2 barrel length right up to the end of the 19th century. This would be understandable, considering the immediate predecessors, the Colt 1860 New Model Army and the 1851 Navy had lengthy barrels. The holsters for the 1860 army could be used for the SAA cartridge gun, and this was no small economic factor. As an aside, there was no specific provision for carrying extra ammunition for the cartridge revolver for many years. Many officers and cavalry troopers were photographed wearing the musket cap box on the field belt, and the local harness makers and saddlers frequently made up leather strips sewn to hang from the saber belt and hold several cartridges.
Anyhow, sometime around the Spanish-American War and the Philippine occupation, the Army brought in a large number of SAAs, had them reconditioned, and the barrels cut to 5-1/2 inch length. I suspicion that this was so they could be holstered in a similar manner to the DA .38 which had recently come into issue. The story of the inadequacy of the .38 in the islands is too familiar to be recounted here. Suffice it to say, guerilla attacks, even in garrison and base camps, were frequent enough that it was desirable for sidearms to be worn more or less constantly, not only for field duty.
Apparently, some of the first of the cut-down SAAs went to field artillery units, and this led to the nickname, "Artillery Revolver."
Best,
Johnny