This is a battery cover located phone/media mount for the Traxxas TQ 6516 with the ability to  add various other mounts. These use the standard 1/4-20 screws used by most standard mounts. You can still access the batteries with this on.
 
TRX4 Sport for example, check transmitter number for confirmation
 
Included is the printed battery cover, the mount arm, 2 stainless steel screws  1 stainless steel nut and 1 stainless steel set screw.