The filler in this pen is similar to the Autopoint toggle, which is easier to see in the video of the demonstrator on that page. Here, the end knob is unscrewed but remains attached. Pulling the end knob opens the parallel bars to compress the sac. Pushing the knob down releases the pressure. The lower bar is also held to an inner ring near the section, and that metal ring is attached through the barrel with a metal stud visible on the outside of the barrel.
end unscrewed 
<- metal stud 
Imprints on both sides of the barrel
