This might be a lightweight aluminum pulley with unique anodized finish for minimal friction and supreme effectiveness. Change components because they show wear and tear to help keep your model operating in top condition. Flange is blue anodized, as the gear part is anodized in a gold color.
Appropriate Chassis
TRF420 Chassis
*The official Tamiya R/C Parts Matching List has plenty of information about framework compatible with Tamiya free and Hop-Up Option parts.