I ordered a new sear housing block for my M&P only to find it won't fit into my older FS with the internal lock.  There is a dimension fit issue - I could grind the new gen SHB down in certain areas to get it to fit, but I don't think I'm going to do this.

















 

Yes, S&W still makes and sells the old "large sear spring" assemblies. If you have a small sear spring assembly, pull it out and send it to them with an explanatory letter - they will send you a large spring version for free. Of course, you may be without the gun for several weeks, so you can also buy one from Midway: LINK. Once you have replaced it, send the one out of your gun back to S&W... when the replacement eventually comes, you will have a spare.