From Mugen:
This alum rebound stop gauge for MBX7R, MBX7R ECO, MBX7TR, and MBX7TR ECO. Made from aluminum, anodized to the original Mugen color with the number markings in both side and Mugen logo. This gauge is very easy to use and read.
B0548 – Rebound Stop Gauge MBX
This weight is to mount on the back of the front gear box and in front of the rear gear box. You can increase the steering and traction at low grip condition and also possible to adjust the balance of front and rear when cornering.
Note: Mounting screw SJG3x8 (2pcs) is not included.
E2420 – Weight 40g MBX7R/7TR
You can mount this weight both front and rear for nitro kit, but only front for ECO kit.
E2419 – Weight 20g MBX7R/7TR