It was based on the Roman numeral "M" representing 1,000, but I don't
know when it stopped being used.

Living in Rome, I should have known... hi.

I have a diagram dated 1953 in which M still represents 1000. In that diagram
they use MF for microfarad (on the other hand a thousand farad would not make
sense).

I still see "M" being used to represent thousands of dollars, and MCF is
still a pretty standard representation of "thousand cubic feet" of
natural gas.

Good to know, I would not have suspected that M$ could mean just a thousand
dollars and not a million dollars. We here commonly use MEuro for million Euro.

Thanks & 73
Tony I0JX