I assume this is regional... I work in accounting and finance in New Zealand (generally following ordinary Western/Commonwealth standards) and I've never heard of using M for thousands. If I used that I would confuse the hell out of everyone around me.
In some areas M is mille as in the Latin/French/Italian word for thousand, e.g.
"It's... a regional dialect."
"What region?"
"Er, upstate New York."
"Really. Well, I'm from Utica and I've never heard anyone use the phrase '100M' to mean '100 thousand'"
"Oh, no, not in Utica. It's an Albany expression."