The original M2/M3?m4 paint was a semi-matte finish. It would "polish" up around user points - and also "blister" a bit in a reaction to oil/moisture from the hands.
The paint on the later M's (M6 LHSA/M6 Millennium and MP) was changed. Environmental concerns and laws prohibited the use of paint with slightly toxic "degassing" properties. The black paint on the later M's are a water based paint - less toxic fumes than the old style that used a different solvent.
The "thickness" of the paint on the LHSA and Millennium was changed. The initial batch brassed very quickly and the pant was later applied in a thicker layer.
The the M6 LHSA was concieved we had a discussion with Leica about the final finish. They claimed that black paint was not a good idea as it would show wear mark very quickly (brassing) and they were quite confounded when we said "Yes, that is exactly what we are looking for.
Initially, the first black paint "new" M was supposed to be the M6 LHSA with the Solms engraving. Leica promised to hold off releasing the Millennium until after the the LHSA version. They did jump the gun though and let the Millennium out before the LHSA.
All of these cameras do brass, both my LHSA 0.85 and my Millennium 0.58 shows it (strap marks, edges of the top-plate, advance lever etc). The MP seems to have a bit sturdier paint - but as it is one of my most used "metered" M's - it now has brassing and scratch marks all over! Not as much as Chris Weeks MP though - it rivals my old bl.p, M2's for wear marks.