Remarkable. Your piece dates to the 1947-48 shift marking the end of WWII and the transition from the Model 1905 Change 4 and the "Pre-Model 10" that followed it. Yours is the first S series I have seen with the Speed Hammer that was popular in the early 1950's. At a glance I'd have called your gun a C series piece. The second million M&P's had V, SV and S prefixes. The third million wore C's.
The switch to the short action hammer occurred just about S 990,000, in spring 1948 a few months before the start of the C prefix. So there were only around 10,000 of these with an S prefix, not very many comparatively speaking.
I don't know how he obtained it, but I know when he was a cotton farmer in Fresno, CA back in the 40s and 50s he wore it with a gun belt that held a bunch of extra rounds. (I inherited the holster but not the gun belt). My grandmother still out shot me when she was in her 80s and passed it along to me.
Welcome to the forum. Nice gun with lots of family history, a treasure for sure. Don’t let the $500 value fool you. Your gun is a high quality hand fitted piece that is better made than most all new guns.