Out of curiousity, any idea how many were produced in 1982?? The serial # is close enough to 8900 that I'd bet it were made in December, and I was born in December of that year. I'd like to figure out how many were produced during the year so I could get a per day average and see how close it is to my birthday.
I know I can order a certificate from S&W with production information for each serial #, do they state the exact date of production??
Serial numbers that year ran from 9D44501 to 17D8900 - I have no idea if each sequence was fully used, i.e., whether they went to 9D99999 before switching to 10D. Further complicating the issue is that the same serial number sequence was being used simultaneously for about 7 different revolver models. However, I think your logic is pretty sound, so if it were me I'd say the revolver was made in the same month I was born - i.e., that's my story and I'm stickin' with it.
I do not believe that the factory letter will give you an actual production date. It may give the exact date that the gun was shipped, but that's obviously not the same thing.