I don't think the letters "mean anything." They just give them a lot more possible combinations. An N frame S&W that starts with AHB was made between Jan and Jul 1985.
Since we're on the subject, I've got a 625-4 that starts with CBCxxxx. Can you point me to where I'd find the manufacturing date range or let me know what that range is? It has "model of 1989" stamped on it but to my (admittedly hazy) recollection I got it new in the late 90s.
Actually, some of the alpha prefixes do have a meaning. As in the gun designers initials. If you look close at this SW1911, you'll see the prefix is JRD. Right Jason...?
A revolver with a serial number that starts CBC was made after AUG 94 and before FEB 95. It's sort of in the middle/late part of the range so around November 94 is about as close as we can get without a factory letter!