This is a style of piston filling fountain pen. The overall goal of this style is to allow the rod to pull the piston back, but then the rod can be disengaged from the piston and slid into the pen. The piston can therefore move the full length of the barrel, maximizing the ink capacity of the pen. I think that the Conid Bulkfiller does this style perfectly. The PenBBS 355 comes close. But if you engage the rod into the piston fully on the PenBBS 355, it is hard to disengage. The piston will just spin in place rather than allowing the rod to unscrew. The Conid has a mechanism to prevent this from occurring. PenBBS has released an improved version of the 355 to correct for the flaw in the original design.
Before there was an improved PenBBS 355, I decided to improve on my PenBBS 355 with my one-off Bayonet modification. I cross drilled the rod and added a cross bar. The new piston is custom made from Delrin plastic and O-rings. I left a slight rim on it to allow it to snap into the fully retracted position, just like on the original. But now it is easier to engage and disengage. You can no longer over-tighten the rod. The simple L shaped opposing slots allow for faster, easier engagement of the rod, and simple disengagement as well. By leaving the threaded head of the rod, I lose a few mm of piston excursion, but the ink shut off feature was left intact and functional.