ADJUSTABLE MASS DAMPER (2.5G X 6PCS) (MINI 4WD LIMITED)
From the Tamiya website:
Mass dampers are a must on courses with numerous elevation changes. This set includes 6 weights for use on mass dampers; mix and match for the desired setup! Also included are 2x25mm and 2x30mm screws, 3mm aluminum spacers, stabilizer balls and more.