[dgacmu]

The way to consistently use 'm' as thousand and 'mm' as million is if you assume a Latin derivation, with 'm' short for mille. https://en.wiktionary.org/wiki/mille#Latin ('mm' thus being 'thousand thousand', since roman numerals don't compose that way -- MM would be two thousand).

That doesn't mean that that's the way it evolved, but it's a way to categorize it mentally so it makes sense. While there's no logic for it conflating with 'million' and 'mega', at least 'mm' is fairly unique, unless you measure your money by its length.

[jegoodwin3]

That's true now but not in all historical periods. In the original system, m. was an abbreviation for mille, and not a composable component of the numeral at all. In effect, it served as a comma does in English text today, to separate the thousands from the units -- both of which were encoded using the composable letter system we are now familiar with.

Later, of course, people assimilated the operation of the M to the other letters, esp. on the dates of printed books. However, if we are discussing manuscript practice in the 15th century, the medieval approach and not the modern approach to Roman numerals would likely be in play...

In any event, using mm. to represent mille mille (a thousand thousand, a million in modern parlance) would create no cognitive dissonance as it does today.