It appears to be a piston or syringe type filler, but it is not. The rod is sealed at the back of the pen, but the internal head of the rod is not sealed at the barrel like a syringe or piston head. There is a breather tube in the barrel. Moving the pump in and out expels air via the breather tube, and pulls ink in on the upstroke. Once full, the pump can be pushed in leaving the barrel full. The pen pictured below has a cherry colored Bakelite barrel. What appears to be a piston seal is just a threaded hard rubber ring that is only there to keep from drawing the piston completely out of the back of the pen. The large rod itself is hollow and wide open at the bottom and will fill with ink, and ink will also fill the entire barrel, as well as up along the side the reciprocating rod.
One way to think of the action on this filling system is to picture the simple bulb filler. Squeezing the bulb is the same as pushing in the Dunn piston. Pulling back is letting the bulb draw ink.