The twisting knob is threaded, but not limited in any way. The clear knob is attached to a sac cut to form a tube which is attached to the section in a normal fashion. Tipping the pen back allows the user to see if there is still ink in the pen. To fill, unscrew a bit to wring out the sac. Let go and it will mostly go back on its own. Tighten the knob to secure.
Why the arrows? 
unrestored pen demonstrating the clear knob.