serial number and production date~!!

[HKperf]

I went to jared's today to look at some GMT's and they told me the serial number has absolutely nothing to do with production year. the manufacture stamps the serial number when they make the case. (information they got from their rolex sales reps)
as far as i know, the first letter indicates the model year and the second number is the indication of the order when the watch was made.
the sales lady told me not to believe the information coming from the forum..lol
i think the serial number has everything to do with model year with exception of late z's and late m's overlap.
but i might be wrong too...information regarding serial would be helpful to rolex buyers~