On clarinets Selmer Paris introduced letters on serial numbers since 1931, starting with L.
Every 10000 clarinets they increased. The M serie dates from 1939 to 1946. On clarinets, then, the letter only serves as a reference for the year of manufacturing and/or the model.
Selmer Paris saxophone's serial are sure longer than 5 digits so it makes sense that they just changed the letter later in history. On saxophones it seems that they decided not to start from scratch every letter serie, but to continue the increasing numbers.
Why and when they changed from N to M is ip to debate, but, considering that the same company did both clarinets and saxophones I'm inclined to think that it means absolutely nothing in term of horn quality...