Hi all,
I live in The Netherlands and I have the intention to buy a M&P 9. I have called some gun shops and only one has a M&P 9 in stock, it has the serial number MPH7xxx, I guess it was made sometime in 2007??
I want to put the Apex FSS trigger kit in it. I would like to ask if there are any problems with a M&P 9 from this era? Is it relatively easy to put the FSS parts in it? I am a complete noob regarding M&P, I have been shooting Glocks sofar.
My M&P9 has a serial number beginning with MPV and was manufactured in 2007. I installed the FSS Kit in it a few months ago and had no problems with the install. Just follow Randy’s videos. It made a huge improvement. 
This kit will install fine in an old M&P, but you would not be able to use the stronger sear spring included with the kit.
You’ll need to buy a new, >2010 mfg. sear block (~27 US$) if you want to use the stronger spring, as the new sear spring is a larger diameter. I believe the stronger spring was included to mitigate failures to reset, as the mass of the FSS sear and timing have changed so much.
I shot my M&P 9mm with the old, smaller diameter sear spring for about ~1K rounds without a failure, but it’s strongly recommended you do not do so by Apex.
With the FSS kit I would definitely get a new or modified sear housing block to use the larger spring. If it was just the normal DCAEK kit you would be fine with the small sear spring.
Thank you all for the information.
Its cheap insurance to replace the sear housing with the larger sear spring/plunger but it is NOT 100% necessary.