I was intrigued, so I did search the CR archives. Seems the letter is a city/province code (some argument as to which), that likely tells you who built it but not definitively.

This is what I found in a CR post by Brian Baylis and note that the complete post indicates these applied to pre-1970 Specials, not necessarily to the GCs

M for Milano
V for Verona
B for Bergamo
A for an unknown city at the moment
P for another unknown city
AV I have seen once and appears to be a Verona frame, don't know why
the "A".

And also this bit of info in another CR post from Brian Baylis:

During this period I was not exactly taking notes nor asking the type of
questions we all might as today if we had the chance to go back in time,
but still many things were discussed (as well as we could through the
translator and other methods) and what was meant by the "MC" was a
question we all knew the answer to. It stood for "Masi California OR Masi
Carlsbad", it didn't really matter. We also eventually found out that the
Italian frames marked "V" were the bikes made in Mario's shop in Verona,
which he explained to us himself. The bikes marked "M" were made in
Milano. No one ever said who in Milano actually made the frames, but I got
the impression that there were deffinitely (sic) more than one and possibly
several builders there. So beyond that, the rest is guesswork. But if what
Masi and Mario both told is is true, I would say that the letters refer to
the subcontractor in one way or another, in the majority of cases.

I included the above for the info that "V" was likely built by Mario Confente's shop...note I said shop. And also to show that there may have been multiple builders in a city. For those of you who do not know, Brian Baylis is a premier builder and painter who did work for Masi in the early US days.

Again, the caveat. That info applied to pre-1970 Masis and may not apply to later GCs. Clearly more people were building for Masi starting in the early 70s.