I have an older M&P first gen and wonder if anyone has a way to check the serial number to confirm the 1/8" sear spring and plunger rather than tearing it apart.
I doubt you will find the information you are looking for from the serial # & doubt S&W could tell you either.
Just take it apart & see with an 1/8" roll pin punch & see if it has the 1/16th" or 1/8th" spring/hole before ordering the trigger/spring kit
IIRC you want the 1/8" sear housing for the Apex Tactical parts
You might be able to pick up the sear housing on eBay or Gunbroker OR?