Like everyone else, I’ve never seen MM or mm in a chord symbol, and I’ve been playing out of fake books since the 70s. But I am genuinely interested in the origin and use of this strange notation and where one might come across it. If you find any examples, please post them.Guys, I just showed MM7, Mm7 and mm7 as the origins of the M7, 7 and m7 shorthand notations as per music theory classes that I took in the ice age.
I don't know where the M7 and m7 notation could have come from otherwise, other than short for MM7 and mm7. And that leaves 7 for Mm as not one of the above so no letter with it. Then mM or m(maj7) is the odd man out since it is far less common.