Anyway, 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 and was
interesting enough to be documented in a tech report.
Only problem there is m6 predates m3 and m4. Th m6(I) page first appears in V2 and there
is a published reference from 1972. Software Tools, from which m3 derives, was 1976, and
m4 was then introduced in V7 (which is also the first research version since V2 without m6
somewhere in the manual.)
Here's a rough history of the m6 manpage:
https://gitlab.com/segaloco/mandiff/-/commits/v6/man6/m6.6
- Matt G.