Actually the thought of it being the original number did cross my mind,but was quickly dispelled as info began pouring in,,my thoughts then turned to that of a presentation piece given for service, the trooper spent 20 years in service with a 10 years as a senior firearms instructor,after that he was the president of the nysp historical division as well as a judge for a great many years,,i was of the opinion it was a custom numbered pistol,he was very well regarded in our community,,hence the interest in this pistol
I'm of the educated opinion that it has been modified or customized seemingly only for the serial number as nothing else is unusual nor exotic, except his name engraved in the side. This was a gun he loved, I would think.
As it seems he was a revered local celebrity status LEO and judge the value in excess of a standard MODEL 39 would be to those who collect such items and likely worth more in that local area, but, still needs to be verified of "what" it is. To others, this would be a detraction form the standard value of an equal condition MODEL 39.
I don't feel this is work of S&W factory quality but you never know, it might be ! I don't think the letter from S&W will reveal anything monumental unless the work was done at S&W and would likely be sometime in the late 1960s or more recent (I think). This feels like a local quality gunsmith or more likely the PD armorer, may have modified this one.
What I can say, conclusively, it is not one of the fabled "missing links" of the pre-release serial numbers that were lower than the first commercial release of Serial Number: 1001.
The only few examples of those are X prefix, with 3 digits, on a variety of frame revisions (one, owned by Richard McMillan with a side access plate) but ALL without model number and all with the short duck tail frames.
I don't think it would be a difficult to have the frame X-Ray-ed. Find an accredited Metallurgist near you, ask him what he charges and ask him to do a few focal ranges, if possible, the first focused on the serial number area.
PS: Just give the metallurgist the frame only, with the stocks removed. Keep the complete slide assembly, slide stop and magazine home.