In their own ads, this is written as either Dr. Faber Self-Filling Fountain Pen, or as Dr. Faber’s Self Filling Fountain Pen. (scroll down or click and see a video of how this “sac mangler” filling system works) A rigid wire spans the pen from the section to the end of the barrel. At the end, it is bent into the shape of a handle hidden under the blind cap. Inside, the wire is bent to cross the sac, and then cross back near the section. To compress the sac, the handle is flipped about 90 degrees, causing the internal rigid wire to pivot. (The two nearly identical pictures below show the extent of the rotation of the handle.) The motion is like a lever type door handle (or flushing a toilet handle). The Hartline Blotter Pen uses basically the same filling system.
Video of the Hartline Blotter Pen filling system shown with the barrel removed.