This is a Traxxas 21-Tooth 32-Pitch Heavy-Duty Hardened Steel Pinion Gear with a Set Screw.
Fits the following models:
E-Maxx
E-Revo
Rustler 4X4 VXL
Rustler 4X4
Slash 4X4 Ultimate
Slash 4X4 VXL
Slash 4X4
Stampede 4X4 VXL
Stampede 4X4
TRX-4 Assembly Kit
TRX-4 Ford Bronco
TRX-4 Scale and Trail Crawler
TRX-4 Sport
TRX-4 Tactical Unit