The Improved Post Pen pictures courtesy of David Nishimura. He states that the two segments are threaded and you pull out and twist to lock them in the longer configuration.
Wallace Pen Co., Columbia S.C. was part of the Ink-o-graph pen company. This example is high quality pen with a branded nib and a good quality feed. It lists a patent by J M Wallace US 1509008 issued in 1924. The pen is a syringe filler with a double telescoping rod. The rod extends and then locks open by twisting clockwise to engage a pin in the small right angle at the end of the groove in each segment (bayonet style). This allows the piston to reach all the way to the section. After retracting, a rotation the opposite direction lines up the grooves and pins and the rod can be collapsed to become short enough to sit under a rather ordinary sized blind cap.
An unusual Reform fountain pen is similar. Functionally it is a syringe filler. But the blind cap can be shorter and the barrel longer due to the unique function of the syringe rod. It can telescope in and out of itself to make is short for storage and full length for filling. (Photo courtesy of S. Chaushov – G-stampo)