The easy part is it means the gun is 1980 or later as that is when S&W began using the three-letter prefixes. The bad news is they were used out of sequence quite a bit.
Possible 1986-1988. Edition 4 of the SCS&W might have better info than my 3rd Edition.
Your Military & Police Magnum would of most likely shipped in....
April - June 1986.
The vast majority of Smith revolvers in this era shipped in an alphabetical manner in well defined and consistent time frames based on their three-letter s/n prefix.