The Lamp Assembly is deeper within the M2 Shock Isolated Bezel then the Lamp Assembly is in the standard Bezel of the Z2. This truncates the surround beam a little bit clearly defining the edge of the beam (you can see the outline of the lens retaining ring) which isn't a feature of the Lexan lens of the standard bezel of the Z2.
Sometimes I can tell that the surroud beam of the P60 in the M2 is brighter than the P60 in the 6P (same bezel as the Z2). Mostly as Dano says, it's practically the same as far as using the flashlight beams go.
The M2 is the tank of the 2 cell models to be sure!
BTW, there is no "dash" in SureFire model codes.
This helps ITI differentiate their M-3 and other weapon lights from SureFire's Millennium Series such as the M3 CombatLight. (sorry for being picky)
Regards,
Al