Over the weekend at the 2019 Chicago show I did two seminars on restoring Sheaffer plunger fillers. Mulling things over, I thought that perhaps a cutaway view of one of the pens would be useful, so I am posting one here. The pen in question is a war time Triumph with the short black section and a black celluloid covered rod. Other plunger fillers are similar, the main differences being the section and inside threads, and what I call the "cartridge" style plunger filler with the removable filler unit.
Going from left to right there are the packing unit threads. You can see the packing unit set into the end of the barrel. Then the alternating layers of rubber and felt (white felt) and the celluloid washer across the end of the barrel in front of the packing unit. The plunger rod is in the forward position, just past where the barrel widens and the vacuum is released. You can see the backing washer, then the head gasket, held on by the nut with the cone end. I don't have a nib in the barrel. While I'm willing to sacrifice a barrel and packing unit, the nibs for these pens are somewhat scarce...
To fill the pen, the plunger rod is pulled all of the way back, then pushed forward until you feel the vacuum let go. Ink is drawn in around the head gasket and backing washer into the barrel. Normal position for the head gasket is a little forward of where it is in the picture, the end of the rod near the packing unit. The tail of the feed pushes on the end of the nut, pushing the plunger rod and head gasket to one side to allow ink to flow easily around the head gasket to the nib.
Note that the section is one piece with the barrel, and the thread ring is held captive between the section and barrel. The black inside at the right is the section, not ink stained material. It overlaps between the thread rings on the right, and where the black ends to the left.
Access to the inside of the barrel is through the front end, with the nib removed.