Paul Wirt fountain pen with unusual wide center portion

These are standard dropper filled pens. The middle potion does not rotate or slide, and it is not part of the filling system. It is a catalogued model as seen below.

This odd pen was found in the catalogs, shown here. The top pen has a three line imprint on the cap and underfeed to match. The spiral case has an overfeed. They are dropper fillers.