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 

16 Tooth

Face width: 8mm

Height: 15mm

Bore / hole size: 5mm

Outer Diameter: 18mm

Pitch: 1.0 metric pitch (M1, Mod1)