This is the Replacement 72 Spur Gear for the Traxxas Nitro Rustler and Nitro T-Maxx Trucks.
Black plastic, slipper clutch style spur gear.
One 72 tooth spur gear
The other parts of that make up the slipper clutch are:
#TRA1994 - tension spring
#TRA4625 - pressure plate
#TRA4622 - pressure rings
#TRA4685 - friction pegs
#TRA4185 - top gear shaft to slipper adapter