I'd like to thank you, Darkwing
, for pointing out the possible connection with the pronunciation of "em". I'd never noticed that before, and certainly it never occurred to me as a preteen way back in the 1970's.
Whether that's the path that Roddenberry actually took or not, and—again—we may never know how he actually came up with it, nevertheless it's certainly one plausible idea of how one concept might have morphed into another during early development.
My appreciation may not have been evident when I pointed out upthread that already by the pitch he had settled on just the letter "M", so I thought I'd mention it.