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 shows a uniquely designed lever and a pressure bar that rides on a short perpendicular 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 alternative style pressure bar, seen on the later lever fillers, is also unique. Rather than a J bar to help it spring back, it has a sideways segment of spring attached to the inside of the end of the barrel (shown well in the patent picture below). Wirt spring activated pressure bar repair shown on this page.
A portion of the original Wirt lever patent with the fish tail or curved fork for hugging a gently curved pressure bar.
Flat tops have the solid narrow lever. More tapered pens have a generic lever. 
Flat tops maintain unvented nib and vented feed.
Tapered pens have standard nibs and feeds.
