3. serves to apply third party camera/QR plates to NN3/NN4/NN5
Normally the Mounting knob stays on the upper rail of NN3 MKII when you take off the camera. It is saved by rubber rings in older versions or with the black plastic washers that came with the set. Insert the knob to the upper rail and then add the washer to the knob. Will keep it on upper rail once you took off the camera.
Think this helps, always a rare feeling if something was stolen.