From a note I took in mid Sept of this year: "The 1st short action hammer was assembled on Oct 21, 1947 s/n S924878."
That was a pre-production gun using the new "high speed" hammer. S&W's nomenclature was "high speed hammer" not the often used "speed hammer" terminology (just to be specific). See the term on a gold box - on the right hand side of the box top. This box is from a K-22, but it is the same on the M&P boxes.
The statement regarding the S924878 comes from some notes by Hellstrom and includes this item: "like K-38 but not swaged at spur."
The actual first
production M&P with the short action was S990184, assembled in the Spring of 1948 , and shipped to Mesbla, Brazil. The date associated with it is April 7, 1948, but it isn't clear whether that is the assembly date or the ship date. I believe it is the latter, since some short action guns were shipping in March. The information on S990184 also comes from Hellstrom's notes.
As others have stated, there was a transition period when guns with serial numbers higher than S990184 showed up with either of the two action types. The lowest number I have so far located with the "high speed" hammer is S990806. It shipped in March, 1948. S990658 has the older long action, as does S991315.
The late S990xxx and a large portion of the S991xxx revolvers were chambered for the .32 S&W Long with the long action, so that creates a large gap in the .38 M&P production sequence (at least as measured by serial numbers). When the numbers shift back to the .38 the incidence of long action guns diminishes significantly, with the vast majority having the short action.
The highest number on a long action .38 in my database is S996765. It shipped in June, 1948.
I hope this clears up some of the questions about the switch to the short action on the M&P revolvers.