Earliest revolvers were stamped .32 Winchester, then .32 WCF (Winchester Center Fire), then .32-20. All are names for the same cartridge. S&W discontinued the manufacture of .32-20 revolvers around 1930. The "M&P" (Military and Police) designation was not used in factory advertising until 1917 (prior to that, S&W called them "Military" revolvers, even though only very few were purchased by the U. S. Military prior to WWII), and to the best of my knowledge, "M&P" was never stamped on any S&W revolver. In fact, until 1957, S&W stamped no model designations on any M&P revolver.