This extending rod portion of this accordion sac fountain pen filling system was marketed as the “new” or the “perfected” model Betzler. The original had an extendable rod as well, but it was a flat piece that flipped outward on a pivot.
There was no filler when I got the large Betzler pen pictured below, and it was too beautiful not to restore. So I made a reproduction of the plunger based on the patent and diagrams in vintage ads. A real one is all hard rubber, but I used metal for the thinnest piece to make it sturdier. Also, you can’t just find a replacement for an accordion sac, and especially not for an oversized pen such as this. To get around the lack of accordion sacs, a large sac with a bespoke compression spring will do the trick. It will control the compression of the sac into an accordion like set of crinkles, and make the sac and filler parts spring back nicely.