The earliest version to my knowledge is the Post Pen, a simple syringe style filling system patent in 1893 US510145 by Woodruff Post.
Later, many 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.
