You bought that postwar M&P at a good price. It is about a year older than you thought. Serial numbers in that range were shipping from the factory in May and June of 1946.
The S on the sideplate and the S prefix in the serial number are related to one another. Here's the story:
In December, 1944, the factory started installing a new safety mechanism to prevent acidental firing if the gun was dropped and landed on the hammer with cartridges in the cylinder. It is usually referred to as a sliding hammer block safety.
This change was made during production of the Victory Model with its V serial prefix. Guns produced with the new safety device had an S added in front of the V. Hence, in 1945 and early 1946, you have SV prefix M&P revolvers, some military, some civilian. Eventually, the V was dropped and approximately 180,000 units were produced with the simple S prefix.
Installing the sliding hammer block safety required some machining on the inside of the sideplate. During the period when there were sideplates in bins both with and without that extra machining, an S was stamped on the outside of the sideplate in the location you indicated to tell the fitters that a particular sideplate was suitable for use on a gun with the new safety device. It was simply a quick visual clue for the fitter.
As inventories cleared out and all M&P revolvers had the internal safety, the S on the sideplate became irrelevant and the practice was discontinued.
The stocks on your revolver are actually earlier than the gun. This style was used from c. 1930 until c. 1941. They look pretty nice.
