"M" should be be used with numbers that are related with intangible and conceptual subjects like money, time, and counting. The "k" is related to the "M"(always a lower-case k) which is the abbreviation for kilo-. 'k' abbreviation should be used with things that are tangible and physical, like distance and weight.
Confusion comes from a former symbol abbreviation of an "M" with an overstrike or overbar. The overbar means that the mille unit (1,000) is multiplied again by 1,000 because MM = M * M in math. A number followed by an M with an overstrike then means that the preceding number is multiplied by 1,000,000.
Since it is very difficult to create this symbol and most people do not know keyboard shortcuts or how to make it, it is hardly ever seen and is improperly abbreviated as a single "M" instead of the proper "MM".
This grammar rule is not very well known outside the financial world and will often be used incorrectly and the slang of "k" for thousand is often used even by venerated publications such as the New York Times and the Wall Street Journal.
So I notice your message under your name says you are trolling, but you actually thought that you were right so you were not trolling in this scenario, just uninformed.
orly?
I can point to any number of formally adopted international standards adopted by accredited standards organizations which are unanimous on the SI system of prefixes. As a serious question, can you point to the same for your financial-based prefix system you describe above?