At some point Wirt gave in and made a lever filler. I have a hard rubber version, but most are celluloid. Around this time, the early 1920’s, Wirt also made the change to celluloid. These are not seen often, probably since the brand was no longer a market leader and the company itself was in decline. The flat top pens still have the ventless nib and vented underfeed. The lever on the underfeed pens is solid metal and narrow, not folded as typical of most pens. So the early Wirt celluloid pens are still unmistakably “Wirt.” The more streamlined pens look like Wirt gave up. They have vented nibs and traditional finned feeds. The later ones have the usual folded metal levers that look like they came out of a generic factory.
Staying true to his desire to be unique, the inside of these pens continues to have a Wirt touch with a pressure bar unlike others in use. The bar is attached to the end of the barrel by a short segment of spring that allows it to be depressed, but then spring back strong enough to hold the lever flush to the barrel. Wirt celluloid pens are uncommon, and seem to be exclusively lever fillers. More pictures on the lever filler page.
A later acquisition reminiscent of the Waterman “moss agate” color.
The nib might not be correct. It is unvented and the feed is a standard underfeed (vs. the Wirt vented underfeed). The pen has the streamlined cap, but the barrel matches the flat top style barrel, oddly with the later streamline era generic lever.. Perhaps it is an intermediate pen?