The original M&P sear was revised by making the sear spring and plunger (and the hole they go in) larger than the original design*. The R mark may identify your example as one of the newer design. This was a running change, I don't know when that change was made. There may have been another minor change to another aspect of the sear.

*Checking the manual, the change to the spring, plunger & hole were made to the sear block. The sear revision was for something else and also was a running change. Possibly influenced by the Apex sear.