Judging by the markings (I assume you quoted from the gun) and the SN, with the key S prefix, I think this gun to be an early post war transtional M&P aka pre model 10. Despite the N frames sharing the S prefix, IIRC they turned over to N prefix well before hitting 900K. I could be wrong about that. I would say the gun probably shipped in 1948 at the latest. It probably shipped in 1946 though or perhaps 1947.
These so called "post war transtional" S&Ws had the features of a pre war gun, with a post war serial range. Earlier features include pre war hammer, half moon front sight, one line company address, "beehive" ejector rod head and perhaps a few other things I cannot remember. These guns came about because S&W did throw away the older parts when they decided to redesign certain features. They used up the already made parts, before the new ones were found on guns. They are interesting because logically, some will have a few of the "old" features while others will have all of the early features. Your gun should have all of the early features.
The "S" was to denote guns which had the new hammer block Safety. IIRC, this was due to an accident involving a US serviceman on a ship, where a S&W was dropped on the hammer, and went off, killing him.
Value? Well thats tough. If 90%+ with original finish and original grips, no box, I would say $400 give or take. More if its a snubnose and more for nickel. Not everyone cares about the post war transtional status, but many S&W collectors do.
