Ok, here you go direct from Atomic in date order
T1 - original Titanium reg 1st and 2nd - no swivel, would need the piston upgrade - must be dedicated
Ti2 - same, original only titanium second and the first could have been brass OR titanium - ditto on the upgrade - must be dedicated
T1x - next version of the T1, new piston and 2nd stage titanium swivel added - must be dedicated
T2 - next version of the T1X- w/ new high flow case design, titanium swivel, new look and the new magnum lever which enhances preformace at depth - must be dedicated
T2x - the lastest and greatest, same features as the T2 only can go back and forth to 40% O2 "out of the box"
there are several conversion kits available to upgrade any of the older ones to be O2/EAN compatible (40%) - will need 2nd stage serial number to determine which one
hope this helps clarify. Any used regulator should be sent for an inspection/service IMHO before use. (by an authorized dealer or factory)
email me w/ any questions.
cheers,
Darcy