Here is a pic of an S prefix M&P that shipped with prewar stocks. The serial number on this one is S814325. It shipped in March, 1946.
There are some M&P revolvers with numbers lower than S818072 that shipped with the postwar
style Magna stocks. Examples include SV807355 (March, 1946) and SV810708 (also believed to have shipped in March, '46). SV809121 is another, although I do not have a ship date on that one.