WASP or Vacuum Fil ink view twist knob

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.