Standard Dropper Filled (or eyedropper filled) fountain pens
To fill the pen barrel with ink, unscrew the section and remove. Fill the barrel with ink using a dropper, and then replace the section. Simple idea. The pen holder (barrel) is the reservoir for ink. The challenge to get this design to work was in the design of the feed, which is the hard rubber part spanning from the inside of the section to the nib.