That is a lightweight aluminum pulley with special anodized complete for minimal friction and supreme efficiency. Change parts because they reveal damage to help keep your model running in top condition. Flange is blue anodized, even though the gear part is anodized in a gold color.
Suitable Chassis TRF420 Chassis
*The official Tamiya R/C Parts Matching List has a good amount of details about framework compatible with Tamiya Spare and Hop-Up choice components.