This design is seen on the Eagle Correspondent Pen under patent 599,591 issued Feb 22, 1898 (and also on a Wirt pen found at the Wirt factory that does not seem to have been produced.) I see this design as an extreme re-positioning of the section joint to the end of the barrel. Picture how a middle joint pen compares to a traditional dropper filled pen. Keep moving the joint all the way back, and this is what you get. Now it also could be seen as a removable and refillable reservoir like a vial, or perhaps a non sealable cartridge.