The internal lock and magazine disconnect safety for the M&P are contained in the sear housing block assembly. Brownell's sells the different flavors of sear housing block assemblies:
940-000-983 (http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=23270&title=SEAR+HOUSING+BLOCK+ASSEMBLY%2c+NO+IL&s=49833#49833) no internal lock - $24.75
940-000-982 (http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=23253&title=SEAR+HOUSING+BLOCK+ASSEMBLY%2c+NO+IL%2c+NO+MS&s=49790#49790) no lock or disconnect - $25.77
940-000-958 (http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=23189&title=SEAR+HOUSING+BLOCK+ASSEMBLY%2c+STANDARD&s=49654#49654) standard, with lock and disconnect - $28.93
The sear housing block assembly (#25 on Brownell's schematic (http://www.brownells.com/aspx/NS/schematics/schemmfg.aspx?schemid=859&m=15&mn=Smith+%26+Wesson%c2%ae&model=M%26P+(Semi-Auto)+)) is a complete replacement unit. The only parts that you will have to work with are the ejector (#30), which fits between the assembly and frame, and the roll pin (#34, which is the same as #12 for $0.25) that secures the assembly and ejector to the frame.
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