I would also like to hear the answer if anyone has one. I came across some threads from a year or so ago, and they were talking about serial numbers of mosquitos. They were discussing how they all had ones starting with A, and heard they were coming out with newer ones that start with an F, but no one had seen them. I just purchased my mosquito new a month or so ago and it's serial number starts with an F.
I would assume that means the newer models start with an F, and the older models start with an A, but that is nothing more than a mildly educated guess.