There were made by many lower end makers and appear in early hard rubber, celluloid, and later plastics as well. A rod pulls on a piston and you fill the pen like filling a syringe. The rod has to stay pulled out, so the pen needs a long blind cap.
The one I keep in my collection is interesting in that it is faceted and has a “word gauge” on it. Unlike many of the lower end versions, the rod is metal and uses a cork seal. Most syringe fillers you will find will use plastic pull rods that take a rubber ring on the inside end.