The Manufacture Nationale d'Armes de Saint Étienne always starting with "F" on a new model........thats an easy one.. the way it was explained to me....each manufacture had a set serial prefix to use.. France did each prefix letter in lots of 99999.. When they got to the end of their prefix run, they started prefixes over again, but with a second prefix letter, so a MAS rifle ...Q99999, the next produced should be FG00001. If it took 3 months or 3 years, that letter prefix ran till it hit 99999. Then there are the cursive and block letters..which cursive change to block was a postwar MAS 36 I believe
Manufacture Nationale d'Armes de Chatellerault (MAC) ... A, B, C, D, E
Manufacture Nationale d'Armes de Saint Etienne (MAS) ... F, G, H, J, K, L, M, N, P, Q
Manufacture Nationale d'Armes de Tulle (MAT) ... R, S, T, U, V
There is also an X series....it's a non firing trainer/cut away/something caused it's rejection.
The above should hold true with all made from 1866 all the way to the MAS49/56, the only exception to the rule seems to be semiauto pistols.