BTW, the names are M6, M7 and M8X.
I think the "X" might mean "escapement" -- the keybed in the pictures is textured and maybe Yamaha incorporated the NWGH from the YC/CP 88s.
I had a short conversation with a GC store manager. CP88s sell like crazy. One comes in and it sells almost immediately. The NW keybed is popular with CP88 people and maybe Yamaha adopted it for the Montage M8X?
All the best -- pj