^ That, If you remove it you can see the little pins on the output gears that the sensor would count- traxxas are just cheap and recycle parts & molds etc.
And FYI ( @ the OP ):
MM = mambamax
MMPro = mambamaxpro
MMM = mambamonster(max)- the 'max' part was dropped officialy, but lives on unofficialy in order not to confuse people between the MM and MMM.
