This is the optional Mugen MBX8/8R Front-Rear Lower “B” Arm Mount from Ultimate Racing. It is machined from T6-7075 aluminum and anodized in dark gray.
It is 7mm thick, and the original Mugen is 5mm. This extra thickness provides greater robustness.
It has been specifically designed to prevent breakage of the arm mount bushing. This is achieved because the bushing rests entirely on the arm mount, so when the car receives a front impact, the pin will never pierce the bushing. Even though this area has been reinforced, a small hole has been left where the bushing rests so that we can see in which position it is mounted.