So I had the exact same question, after being given a schematic that my retired colleague had designed on a version of orchad running on windows 2000, so obviously that PC doesn't exist now so i hadn't a hope of checking the parts list. Anyway after a lot of time on google and other forums I turned to another colleague of mine with considerably more experience than me.. He explained to me that this is the way uF were sometimes represented in old computer programs as the 'micro' sign was not available, I guess this was before extended ASCII (why they didnt use 'uF' like i just previously used i don't know). Any way it breaks down like this:
if 10^3 pF = 1nF (written as such, i presume);
10^3 nF = 1000nF = 1 microFarrad, which was written as 1M
so it stands to reason that M1 is 100nF.
So in summation my colleague explained to me that the 'M' in 1M can stand for 'micro', but to now confuse things he also carried on to explain that;
10^3 microFarrads = (1 milliFarrad) = which would have been written as 1G.
So the other bizarrely labeled capacitor, in my circuit schematic, labeled '2G2' represented 2200-microFarrads.
This brings me to think that maybe a 1M capacitor could be thought of as analogous to a 1M resistor where the unit is Ohms and the value is 1,000,000 (million) ohms. Except that in the case of the capacitor the unit would be pF (picoFarrad), which would fall in line with the standard way of labelling small value capacitors. I.e a '103' capacitor means 10*10^3 picoFarrads (or 10+3 zeroes, same thing).
Hopefully this helps anyone else who finds this forum page searching for a similar answer. Also, just to add some more letter number combinations for capacitors i have come across;
22J = 22pF (pico Farrad), here the J just represents pF very simple. So 2J2 = 2.2pF.
4n7 = 4.7nF (nano Farrad), like the J, except here the 'n' just represents nano.
If any one disagrees please let me know. I believe it may be a convention but just not one i've ever met before.