In last ~couple years have run across a few references to Type M2, or M3 dane axes. I don't recall seeing M1, or M4 etc. In many years here I only recall plain type M discussed. I recently re-skimmed Petersen & did some searches here on mA & didn't spot anything except plain M.

Is there some newer "Mx" typology? Does anyone here know the difference between M2 & M3? Thanks.

AIUI, type M2 and M3 are proportionately taller/wider than M1, and M3 features a prominent ridge right behind the edge bevels.

See here for illustrated examples, or here for a handful of photo references.