From Tamiya Japan site:
"1/10 RC TB evolutions III
In installation of front one way and rear ball diff. being attached
Explanation drawing 10 in " the assembly of the body ", the occasion where one way and the ball diff. are set to the body front and back, it can designate the opening about of the 1mm as 1510 bearings and the A3 part. This is the opening with respect to the structure which is necessary in order to hold down the bearing with the A4 part.
Shaving this part, when it tries to be able to set the bearing without the opening, fixing of the bearing to become insufficient, because it becomes the tooth flying of the gear and the cause the gear licking, please avoid processing."