The gear was made of  cased hardened 4340M Steel then nitridin the surface.

 It's has very tough core inside and high hardness on the surface for wearproof. 

Specifics 

26 Tooth

Face width: 8 mm

Height: 15 mm

Bore / hole size: 5 mm

Outer Diameter: 28 mm

Pitch: 1.0 metric pitch (M1, Mod1)