once you've got M3 and M4, you've got a
naming convention; I'd
think it a safe bet that there was an M5 that was an internal
experiment, and that M6 was simply the next in line
M6 came first, created by Andy Hall as a portability tool for Altran.
I always assumed the name echoed Ken Knowlton's L6 (BelL Labs low
level list language, with a superscript 6). I seem to recall that M6
was endowed with a very labored acronym.
Doug