Using an A19 (Black type II anodised) body extender to extend the body of an M2 then requires the use of the P90/P91 incandescent Lamp Assemblies or the P60L LED Lamp Assembly powered by the now three-SF123A batteries.
An M2+A19 is more like a "C3" rather than an M3 (the M3 uses a completely different shock isolated bezel and Lamp Assembly design compared to the M2/C3 etc)
The original P60 Surefire lamp will fry if you use 3 cells. If you want to stick with the incan surefire the lamp is either the P90 or high output P91. Lumens factory also has some nice lamps. The SF P60L runs fine on 3 cells. And yes a M60 runs fine off of 3 cells as well. You just want to make sure that the input voltage of the lamp and the cells your feeding it match up or you run the risk of damaging the circuit or the lamp itself.
gotcha - how inferior are the G&P extenders over the surefire A19s?