Here is an unnamed pen with a piston or syringe filler with a unique feature. The piston is controlled with an external collar attached to the piston rod by a pin. I suspect that it is from early in the screw cap era based on the imprint telling the user to unscrew the cap. I have seen a similar pen on ebay (same filler, different chasing). It was imprinted Unity No 9 Selfil-safety [imprint shown at bottom of page]. Calling a screw cap a safety pen also dates the pen in the early 1910s.
5/2021 update: I acquired another sliding collar piston / syringe filling pen, decidedly lower quality, but with the same mechanism. It was apparently (according to the box) sold with a plated nib, and was designed to take other replacement nibs.
5/2023 update: Same idea, but with a short 90 degree dogleg in the slot to help keep it in the open position. “Ricker” branded pen with 18K nib.