Here is what Bob Hovey has to say on the subject. Used with permission.

Letter prefixes on Masi frame size stamps meant different things at different times. During the Faliero era (say '72 and earlier), it was usually an "M" (some have suggested "Milano," but almost certainly "Masi"). However, there were other letters which supposedly designated the city or subcontractor of origin (Masi farmed out a lot of frames during the bike boom era). For example, we are almost completely certain "V" was Verona (Mario Confente's shop). "B" could be Bergamo. There are quite a few "A" frames from the early 60's, most of them Nervex-lugged and as far as I know all sold in the US, so I'm curious if these are VeloSport imports and for this small group at least, there's an outside chance that "A" might mean "America" (to be clear, this is purely my own speculation). Several early '70's "AM" frames have turned up, I've often wondered if these were Alberto's early frames when he first started building (one notable exception of course, "AM58" built for Italian champion and national team coach Alfredo Martini in the early '50's). There are quite a few "P" bikes, and a large number of them are pistas, so I suppose there might be some connection there.

For a time, Alberto and Faliero were somewhat reticent about the meanings of these letters, but a few years ago Alberto told me (through a translator) that the bike boom era letters were indeed city designations for subcontrators. As for the pre-bike boom years, I suspect that the letters may have had numerous meanings... they might have been subcontractors, but they might also designate team bikes or a personalized designation for special customers.

Beginning around 1973-74, Alberto began using a two letter prefix that represented the initials of the person or bike shop which ordered the frame. This, along with his date codes (YYMM, in a smaller 1/8" stamp) provided enough variation to insure that there would be almost no frames with duplicate markings, without having to use consecutive serial numbers the way US bikes did (which can be hard to keep track of... see Rob Roberson's build list on my site, there are several duplicate series of numbers). The date code didn't begin at the same time as the double letter "Customer Code," I think it started a bit later, around 1975. In 1974, some bikes were just stamped "4," and prior to that (beginning in the late 60's) a date was out of sight on the steering tube (and was not always a good indication of the production date since forks were often built in large batches months ahead of time, and might not even be by the same person... for example, Confente began his work with Masi by just building forks, so occasionally you'll find a bike with a "V" fork and an "M" bottom bracket).


I am still looking for info on the "S" prefix.