The pressure bars in Wirt lever filling pens are unconventional. The bars are attached to a plug that is inserted into the end of the barrel. The earlier design has a floating bar anchored to a perpendicular pin at the end of the barrel. The later version is designed so the bar springs back up against the lever.
The pen pictured below came to me with a standard new replacement J bar. The bar did not sit up against the lever well, so the lever jiggled a bit. The original bar was missing and a corroded remnant of the spring was visible at the end of the barrel, still attached with the cross pin. After removing the replacement pressure bar, a wide dowel was pushed into the end of the barrel, and the plug was loose enough to come out without risk to the barrel. The original partial spring and pin are shown in the first picture, along with the new spring.
A new spring was fashioned using a piece of music wire of comparable diameter, and this was soldered to a new pressure bar cut from a J bar of a junk pen. The cross pin to hold it was also corroded, so I cut a new one from a thicker piece of music wire. A little test fitting and bending to be sure the bar would be seated parallel to the barrel and we were good to go. The original design was restored.