Sorry if this is heavy, but,
I don't know about everyone else, but the
M Series is
SureFire. M for Millennium Series "Special Operations Lights" -
M1 InfraRed Illuminator
M2 Centurion
M3 Millennium CombatLight
M3T Millennium CombatLight
Turbo
M4 Devastator
M6 Millennium Magnum Assault Light
M1xx Millennium Handgun WeaponLights
M5xxA/B Millennium Fore-end WeaponLights
M900A/B Millennium Fore-grip WeaponLights
MU9x__ Millennium Universial Tactical WeaponLights
Pelican have their rechargeable "Black Knight Series" -
M7, M9, M10 [alkaline 4C] & M11.
ITI have their "Tactical Illuminators" -
M-3, M-4, M-5 & M-6.
All these require two 123A Lithium batteries.
A lot of "gun people" take "M3" to mean the M-3 from ITI but I believe the "M3" can only be the SureFire M3 Millennium CombatLight.
Alastair's rant is over (!)