Actually, $1M historically meant 1,000. MM (1,000 x 1,000) meant one million.
There was a law case, where 5 million somethings were ordered by mistake instead of 5 thousand because of the extra M. The court ruled that since the customer had a business track record, the seller should have known better.
Both M and MM are deprecated because of the confusion according to this site:
https://www.ismp.org/recommendations/error-prone-abbreviations-list
Yep. I was hired as a plant engineer for a paper company way back in the dark ages of my youth, and was confused for the first few days. Where I used “K” for 1,000 in other places, the paper company used “M” for 1,000...