Something else to bear in mind is that the engineers have made some revisions to the frames and sear housing blocks over time.

We were told in my last armorer recert that the newest SHB's wouldn't fit into the older frames, as the newer molded SHB's wouldn't align with part of the older frame mold/cuts.

I confirmed this when trying to slip a new 9/.40 SHB into my 2010 production 40c. It wouldn't fit (and I'm not interested in trying to modify the frame).

However, I also had a couple of older SHB's of the MA-compliant type, which already (originally) came with the larger sear plunger & spring, and which fit into my older production guns. (That was part of how the MA-type guns got heavier trigger pulls, with the rest being accomplished via a different trigger bar tail profile and a heavier trigger spring, I was told.)

We were also told that when the new guns came with the PC sear (as part of the new enhancements), that use of the H trigger bar provided for better engagement with the sear.

I've been told that S&W has changed how they look up compatibility of M&P pistols parts, BTW. (Found out when I needed to order some more parts as an armorer.)

Instead of the previous "general" model parts listing, they now look up parts needed for any particular pistol by its specific serial number, to make sure any parts needed are going to be appropriate for that gun. This apparently more easily allows them to keep up with any revisions or production run changes for different guns.

I'd imagine this will allow the parts dept & repair techs to keep track of which vintage/design of SHB fits whichever frame.

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Ret LE Firearms inst & armorer