Hi, Ron,
I am going by the "Standard Catalog". It says, under ".38 Military & Police - Pre-Model 10", the following"
"A contemporary source indicates the S prefix postwar guns began as early as December 27, 1944 at around serial number S769000. Others report Sept. 1945 beginning at S811120. It is reported that at S990184, Apr 7, 1948, the new short action was introduced, becoming the now familiar Model 10 in 1957." Since I have seen postwar M&P's with the "S" prefix (no "V"), I certainly know they existed.
FWIW, I question the S811120 number because I suspect they simply began the post-war serial numbering where the Victory Model left off, which would probably have been a bit higher. They stopped stamping the "V" but retained the "S" for the new safety. (Neither the V nor the S was ever actually stamped as part of the serial number; the letters were stamped separately before the gun went to the numbering machine.)
Anyway, the presence of the "S" and lack of the "V" indicates a postwar gun. Pre-war guns had no prefixes.
Jim