A pen like this was posted on the large Facebook group 7/24/2020. I have never seen it before, and I was unsure if I would ever find a vintage one. So, I decided to make one to represent this style in my filling system collection.
The pen is a standard sleeve filler fountain pen or thumb filler fountain pen with a cut out that exposes a bar over the sac. These need a cover for the hole to prevent accidentally pressing on the sac. Rather then rotate or slide a cover for the opening in the barrel, there is a freakishly long cap that covers the hole when the cap is on, as well as when the cap is posted.