Sai che circolano da anni molte notizie al riguardo.
Io mi sono rivolto direttamente al produttore, ottenendo da Selmer Communication la risposta che segue (che sono autorizzato a pubblicare):

To be honest, nobody knows !
A lot of rumors, but nobody knows the real reason or explanation, and absolutely no information in our data about this modification. At this period, this was not important for our manufacturing (like many other things that are of particular interest to collectors but have no particular meaning for us or our manufacturing)

It seems that the most probable reason is that the letter was modified in the same period that a part of our trademark registration place on the logo. No specific signification about M or N. Probably M for Mark as we can imagine and N for Number.
But really, no specific importance.