IIRC, "M" grooves are simply a pair of micro V grooves in a U groove. Confused? You could almost say it's a W groove, really, as it illustrates the point better.

In other words, rather than coming to a point, or having a flat bottom, the middle of the groove is raised slightly in an M groove.

Better? More spin? I don't know.

I may even be wrong - but I think I'm right.