Hi all,
Just caught this topic.
There are a few reasons why standard M's and M15 keys are not that compatible but here are the two most important ones:
1) As is evident in the OPs photos you can see that the M's stem is longer than the M15's. That right there is the main reason.
2) Offset, the M keys have a different offset (front to back) than the M15 keys.
So what exactly can you do with them in stock form you ask?
Well you can put either one on either keyboard, but... only one will likely work
M15 keys don't work on M's without modification
M keys work on M15 boards but they are hella loose and fugly but will register keypresses, with some occasional binding. They are too tall and they twist more than Chubby Checker. They will also sit too far forward and likely wont comfortably fit the "top" cap. But the inner stem cap will fit, but again they will be leaning/shifting far forward. M keys are also bigger not only in height but in girth in general. It's footprint is larger.
For the TL;DR crowd
M15 on M, LuLz, turtle head stance, no dice.
M on M15, if you're into psychedelic drugs go for it!