Twoeyes:
Your gun with the S prefix should be a transitional M&P from around 1948 or so.

Mr. Rush:
Referencing your excellent post at the bottom of Page 3, the Peters .357, I submit one more version of the same ammo. Notice this box has the Bridgeport address on the bottom right corner instead of spread across the entirety. Lead bullet, nickel cartridge case not referenced on the box, small primers.
Ed