The S in the SV prefix and the S stamped in the upper corner of the sideplate both indicate the presence of the newer sliding hammer block safety. This change was approved in September, 1944 and production of the improved guns began in December of that year. The first shipments to the military began in January, 1945.

The S on the sideplate was put there to indicate a sideplate that had the additional machining necessary to accommodate the improved hammer block.

The hole in the frame just below the thumb piece is threaded to accept the hammer pivot stud, which has matching male threads and is screwed into the frame from the inside at that location. You can see the rounded end of the stud on your revolver. On January 18, 1946, an order was issued to replace the threaded hammer stud with one that pressed into the frame. Once that change was implemented, the hole became virtually invisible, since the new stud was pressed into place and polished off flush with the outside of the frame before bluing was applied. Revolver #S819462 is reported to be the first M&P to receive the new design. Guns in that serial range were shipping in June, 1946.

I did notice in your pictures that there is no evidence of a plugged swivel hole on your revolver. At SV812xxx we would expect a hole, but once in a while there isn't one where we expect it to be.

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Jack
SWCA #2475, SWHF #318