A very popular system for filling pens used in the ‘20s. Remove the cap from the end of the barrel and push the button that is attached to a flexible piece of metal and the ink sac.  This will draw ink into the sac.  Parker’s pressure filler worked by dipping the nib and part of the section into the ink and pressing the button on the end of the barrel (revealed by removing the protective cap on the barrel end), counting to ten, and then letting go. The cap was screwed back on and the user was ready to write with the pen! Push fillers work the same way, except you don’t have to remove the end cap.