I just received a new mag catch for my M&P 9 Compact from S&W, along with the new style striker. You might look at you mag catch..there is a very small number 1 inside a circle on the side of the plastic. You have to remove it from the gun to see it.

There is also a new style striker for the M&P's because the original ones had reports of breakage. Info on the M&P Forum for that as well.

The part numbers on my packing slip that came 3 weeks ago were:

27789000, Magazine Catch, Rev. F

279770000, M&P Striker Assembly, Rev. B

I replaced both the striker and mag catch on a four year old Compact...but never had a problem with the gun....just wanted the latest revision parts.