53421 Aluminum Pinion Gear 42T/43T OP-421 - Tamiya  [53421]

That is a couple of Aluminum Pinion Gears for TA-04 framework vehicles.
Here is the gear that attaches towards the motor.

CHARACTERISTICS: Aluminum construction

INCLUDES:

• One 42 enamel pinion gear
• One 43 enamel pinion gear
• Three 3mm set screws
• One allen key

REQUIRES: Installation onto vehicle.

SPECS:

• 43 enamel gear diameter: 18.7mm (0.74")
• Pitch: 0.4 M (48P metric)

COMMENTARY: It is possible why these gears may focus on other 48P vehicles.