RS-1 Upper Rotator Assembly Kit. This kit converts the upper rotator of NN3 to clamp style rotator in affect converting NN3 to RS-1 with a NN3 base rail which is longer than the RS-1 (slightly shorter lower rail).
This allows you to customize a nodal rail that suits your setup. E.g. The SEL1635F4Z (from Sony) has a Entrance Pupil (Nodal point) beyond the upper rail of the NN3, and so if used with this, allows you to acquire a longer nodal rail (for the upper section).