The DNR-120 Nodal Rail from Desmond is a 120mm upper rail that when used along with a panning head, a vertical rail, and a lower rail will make up a complete panoramic head.
Precision scales on the sides of the rail and the clamp allow for micro adjustments so that you can find your nodal point for each lens. This prevents you from having to make timely adjustments the next time you use that lens. A bull's eye bubble level on the rail ensures your camera is level on the horizontal and vertical planes.