M is 1000 in Roman numerals. While it is uncommon, M is used to mean 1000 in business contexts, such as CPM: https://www.investopedia.com/terms/c/cpm.asp

Therefore "mm" is the most unambiguous way to denote millions, and is used in accounting.

It’s used a lot in finance and accounting, and stems from the Roman numerals. It’s also used in advertising, CPM = Cost Per Mille.

"mm" is a perversion of "MM", which is derived from Roman numerals (M is 1000, and MM is M*M). This notation has a long history of use in business and accounting.

For consistency, the author's use of "bln" should be "BB", or at least "B". These have no Roman antecedents, but they are common in accounting.

I've never seen "MMM" for billion. Missed opportunity! I'll have to suggest that to our CFO for our next filing.