This is only seen on the Moore pen. It was advertised as the “Moore’s Non-Leakable”.
There is a simple round seal at the end of the barrel with a rod that extends and retracts. Simply push to extend the barrel. It is made more elegant by covering the rod with a sleeve that slides over the barrel with extension or retraction of the nib. The pen is shown fully assembles and also with the sleeve removed to show the simple push rod.