"S" prefix serial numbers were used on K-frames for a very short period immediately following WW2 - they ran from S811120 in 1945 (SCSW quotes but apparently disagrees with another source claiming S769000 in 1944) to S999999 in 1948, so your .38 M&P (pre-10) dates from that time span. "S" models in very good or better condition are said to bring a 25% premium over their non-"S" counterparts. Based on the SN, your gun was made around the time that S&W was switching to the short action and new hammer profile and the book also mentions a premium for these so-called transitional models.
Numbers in various places that are obviously not the SN are called "assembly numbers" and are used to keep track of fitted parts during the manufacturing process. They have no further significance.