The "S" series K frames where manufactured between 1945 to 1948 and are considered Post War M&P's.
That serial number is most likely 1946.
I own 2, S857xxx and S869xxx, a tad later than yours, but not much. There's a lot of information about them here on the forum if you search for it.
There's a member here, JA@AK, who is compiling a data base on the "S" series K frames, to track the changes between WWII and 1950 where the design became what is referred to as the "Pre Model 10".
Love to see some pictures.