The Chilton fountain pen utilizes pneumatic pressure to compress the sac. Pull back the barrel to expose an inner metal tube containing the sac. There is a seal between the barrel and the tube. The end of the barrel has a hole. Cover the hole with your finger while pressing down, and the sac will be compressed by pneumatic pressure. Then release the hole to let the sac fill.
The lower end “Chiltonian” pulls back on the metal rod instead.
Remember the blow filler? Here it is without needing your mouth. Extending the mechanism admits air inside. Plug the hole and press, and the pressure will squeeze the sac. Much better than using your mouth.
