Most (but not all) of the M&P's are cut for the ambi-safety levers. You can buy plugs to fill the holes on those models. I have one M&P with the plugs and it runs just fine. The plugs are not going to move or fall out without some serious assistance.
Those frames were made for ones that have safety levers. I've never seen one from the factory like that though. I took my safety off and bought these filler blocks aftermarket to close the holes. Not quite sure why the factory would do this.