This is ARC R8.3(E) Adjustable Front Shock Base.
The adjustable rear shock base is the latest option part designed for the ARC R8.3 and R8.3E. This allows the shock angle to be adjusted by adding or removing shims.
Compared to traditional shock towers, racers can adjust the shock angle in much finer increments to better fine tune the handling of their car - with more shims below the shock absorber standing the shock up and making the suspension harder.
The new shock base also features a 3mm carbon brace that bridges the left and right sides of the shock base. This brace can be added or removed to adjust the flex of the rear bulkhead. With the plate installed, rear side flex is reduced resulting in an increase in rear end stability.