A:
The correct nomenclature for the shoulder holster in question is "Holster, Shoulder, Revolver Caliber .38, Colt and Smith & Wesson". It is intended to carry the 4 inch barreled Victory and Colt Commando revolvers, although the government drawing states that it is also suitable for 2" and 5" barrels. The 5" will protrude through the bottom of the holster by one inch.
It is very similar to the M3 holster used for the M1911A1 .45 pistols, so among collectors it is often referred to as "M3 style", but it is not a true M3. Some contemporary references refer to it as the T-2 holster, but it is not clear that this designator was ever officially adopted.
The M7 shoulder holster has two straps, one for the shoulder and one for the chest, and was designed for the M1911A1 pistol as well. Later versions can also accommodate the Beretta M9 pistol.
HTH.