The .45 sear housing blocks have different parts numbers than the 9/.40 sear blocks for a reason.
Last I heard, APEX was doing their own drilling/machining to use
their springs & plungers in older M&P sear blocks. You'd have to ask them about their parts.
The factory started switching over to the heavier sear plunger and spring that was originally designed for the MA compliant guns (meaning they will add a bit of weight to the trigger pull, and they certainly did so in my couple of guns).
S&W is restricting sales of the factory sear blocks, both the individual parts and complete assemblies. I was told at one point that they want agency addresses used for shipping sear blocks/parts, even to LE armorers. (It was suggested that their legal dept is concerned about potential liability, what with all the owners doing their own "filing, modifying & improving" ... and then having problems.
They're still selling parts to outside vendors, but I haven't tried to keep track of to whom.
I've heard it said that there's a division among the S&W corporate people regarding whether or not (and how) to continue selling a lot of parts & assemblies for their firearms to regular owners, and if so, whether they should start offering instructions for installation of those parts & assemblies that could be relatively easily done by owners. Up in the air, so to speak. We'll see.
FWIW, this is the difference between the original sear plunger & spring and the newer (MA-type) parts, as used in the
factory sear blocks. The springs & plunger are obviously not interchangeable.
I used the original sear blocks in my own M&P's for several thousands of rounds, and only changed over to the newer ones to see what differences might be felt (trigger pull). I know other M&P armorers & owners who are still using the original sear blocks (with smaller plungers & springs) without any problems (or concerns, but then they maintain them and don't allow the sear blocks to get filthy and contaminated with excessive solvents, CLP's, fouling, etc).
If you ever start to have any issues, contact the company and let them examine your gun. If they feel it necessary to replace the sear block, they will.
