Syringe filling pen with two segment rod. Parts connect via threaded portion.
The Improved Post Pen (hard rubber rod version, pictures courtesy of David Nishimura).
This is an early syringe type piston filling pen. It has a long blind cap. To use the piston, the inner rod is extended and locks to the wider portion via a small threaded area inside. This would make it much easier to use the piston while minimizing the length for storage.
The pens list Patented April 1901. I am unable to locate a matching patent. The original patent US 510145 in 1903 is for a single piece piston/syringe rod.
The example in my collection has a metal piston rod.
