These are the first filling systems to know about:
Early pens
The “pen” was what we call the nib. The rest was the “pen holder”. The barrel was eventually made hollow to hold ink, so the “pen” had its own “fountain” of ink.
Simply unscrew the pen, fill the barrel with ink using a dropper, and reassemble. This is called a “dropper filled” pen. But most people call it an “eyedropper” pen, or even abbreviate it as “ED pen”.
“Self filling pens”
Levers mostly, with honorable mention to the crescent filler
It was an exciting innovation to no longer have to carry a dropper to fill a fountain pen.
The earliest ideas are more similar than different. Inside the barrel is a long narrow rubber sac. The side of the barrel has a device that allows the user to compress the sac. With the nib submerged in ink, release the compression on the sac and it fills with ink. Both the lever and crescent fillers went to market c1900-1905. The lever became much more popular. Sheaffer is credited with inventing the lever filling pen. Others found ways around the patent to release their own levers. Conklin invented the crescent filling pen, and maintained the patent. It was harder to imitate without violating he patent, and people likes pens without “stuff” sticking out of them. So levers win.
There is a long flat metal bar alongside the sac. Lift the lever and the inner part of the lever pushes the bar against the sac. For a crescent filler, the crescent shape is attached to the bar. Unlock the crescent by rotating the locking ring, and push to compress the sac.
Let the nib stay submerged for about 5 seconds to allow the sac to fill. It is not instant.
Parker focused on this version. There is still a sac and a bar alongside. However, the mechanism is a button on the end of the barrel. You need to press rather hand to get it to flex and compress the sac. The advantage is a beautiful pen without visible mechanisms. The downside is a cap on the end “the blind cap” that must be removed and replaced to fill the pen
In particular, German pens like Montblanc and Pelikan used a different system. The barrel has a piston. Twisting a knob on the end of the pen makes the piston slowly move up or down, so it can be used to pull ink into the barrel. Some pens have a knob under a blind cap, and some knobs are part of the end of the barrel.
Intermediate Level Filling Systems
These are easier to demonstrate than explain! Follow the links!
Vacumatic (by Parker)
Squeeze type converters, which is the same as Parker Aerometric (and many more “thumb” or “sleeve” fillers)