You are the Man Andy. Don’t be so modest.Not an official answer. Note and rendered without going into the attic to look at SL-GIANV series of RB.
Guess: It created an easier way (condensed and singular/plural) to address these terms...I think the OP would likewise include half squads, which was a later element, thus addressing why MMC and HS had to become divisions which would then parse out SMC as an extension.
All that said...it is an assumption on my part...and there are far* better SL / ASL historians out there than myself....
The stated and intended purpose of the ASLRB was to codify everything in the game with very specific language and terminlogy so that it would close loopholes. In the original, "half-squads" weren't part of the original rulebook. They were introduced in COD and the first counters were in GI: Anvil of Victory. So whenever you had a rule in the first rulebook allowing a "squad" to do something, there was a question later on - does that include crews? Does that include half-squads? What if I have 3 scout counters and a sniper? etc. So the term MMC could be used to include everything - squad, half-squad, crew, or equivalent in SMC. Much easier to write the rules that way, fewer special exceptions.Any idea why Avalon Hill (or MMP) departed from John Hill's terminology of squads and leaders and called the MultiMan counters and SingleMan counters?