With regard to the first stages there basically isn't any. Us Divers had a habit of changing the number when they made changes to the ports sucha sgoint with 1/2" LP ports rather than 3/8" LP ports or changing the number, angle or orientation of the ports. They also were known to slap a new number or name on a Conshelf first stage when attaching it to a new second stage and the Pro Diver is a good example of this.
The following US Divers first stages are identical to each other in all aspects other than the number of LP ports and the sizes or orientaton of those ports:
Conshelf XII, XIV, 20, 21, 22, and SE2 and SE3.
"Supreme" in the name of the first stage just refers to an optional cold water kit that can be retrofitted.
With regard to second stage, there is very little difference. The following second stages are in groups that are mechanically identical:
Conshelf XIV and Conshelf 30
Conshelf 21, Conshelf SE2, Conshelf SEA, and Pro Diver
Conshelf 22, SEA, SEA4, Conshelf SE3, Pioneer and Aquarius
Arctic
Aqualung still fully supports all of the above regulators, so getting parts is and service for them is not a problem.