If I recall correctly;
there are several versions:
MKI: came out and had issues with waterproofing (too few o-rings installed) and the clicky-switch-cap was a tall one, also the parasitic drain was still very high.
MKII: came out shortly after the complaints were heard and this one came with extra o-rings installed. Also in this series, the clicky switch and tail cap for it was redesigned (now being a lot flatter)
MKIII: (introduction of the M2XS4) came with a few more improvements, the improved user interface with the hidden modes, less parasitic drain, fewer crenelations for the SS bezel and screw on diffuser now came standard (also increased the RSP by a tenner) with the light. Oh, and I think this M2XS4 was the first of which the head could be unscrewed by the end user.
If you get a MKI/II and you don't like the strobe after max, do the shim mod as many have done. Do it right and the strobe is only available with a very hard turn like mine does