Wirt had his own lever patent. The lever on Wirt pens, until the rather generic final pens, is quite narrow, solid metal, and the inner portion ends at a sort of curved fork or fish tail design that is distinctive. Some have a small “W” on the lever while others appear to be plain. The early patent showed this uniquely designed lever and a pressure bar that rides on a bar at the end of the barrel. This is the bar on my example that also has the roller ball clip that appears to be the same as that used on Wahl Eversharp pens. The pressure bar on the later lever fillers is also unique. Rather than a J bar to help it spring back, it is a sideways segment of spring.
A portion of the original Wirt lever patent with the fish tail or curved fork for hugging a gently curved pressure bar.