I might guess that the round knob is actually intended to replace the threaded knob on a tripod head.

It may actually not be part of the Quick-shoe, but a part of another tripod instead.

The diagrams I have seen of the Quick-shoe 2 don't show any such knob, but then they dont show more than on outline of the item - they can be found on various "system" brochures, and the manual for the original RB67 Pro.