I'm an armorer for M&Ps and while I don't know what that piece is, it isn't a part found in a standard, non-magazine-disconnect, non-frame-safety version.
Short version: you un-California'd it.
ETA: just checked the manual. That is the trigger bar block. It is held in with a spring on the right side of the sear housing, and its only function is to block the rearward movement of the trigger bar when the magazine disconnect is installed and no magazine is in the magazine well.