I hope you got paid $150mm to create that.
Personally I hate that notation and use capital M whenever given the opportunity. I feel like it used to show superiority or present another barrier to people outside of finance looking in.
a millimeter is = to 1000 micrometers.
1 thousand 1 thousands = 1 million
Disclaimer: I've been in the US my whole life.
And don't forget that 'cents' is short for 'centidollars'.
All of the SI prefixes k and below are lowercase.
I suspect people use MM/mm for the same reason, some guy just made a typo one day and it caught on.
(from the UK where m = million)