Round butt stocks are all over the place on the postwar Military & Police revolvers. Some shipped with hard rubber round butt stocks; some with 1930s style walnut round butt medallion stocks (like the OP's); others with the "new" round butt Magna stocks. I've seen all combinations. But, it seems to me I've seen more with either black rubber or the 1930s style than with Magnas, when the stocks number to the gun or are confirmed by a factory letter. My guess is they had a lot of the 1930s types hanging around after the war, and, unlike the square butt prewar Magna, they did not run out early in 1946.
I know there is an official letter floating around out there that says definitively that there were no prewar Magna stocks at the factory when the war ended, but the predominant stocks shipped on the earliest S prefix guns were clearly the prewar style. They shipped pretty consistently up to the S815xxx range with a few examples in the S816xxx range.

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Jack
SWCA #2475, SWHF #318