Unusual Paul Wirt Fountain pen feeds that are rarely seen
For all but the more ardent collector of Paul Wirt fountain pens, there are three styles of feeds: The two-piece Paul Wirt feed, the Paul Wirt paddle overfeed, and the vented underfeed. However, there are also two feeds that are rarely seen that form the conceptual bridge between the paddle overfeed and the vented underfeed: The over and underfeed, and the two piece vented underfeed.
[First a brief mention of a feed that is unusual, but not a missing link. The feed on the rear end joint dropper filled Wirt pen is a tubular over and under feed that resembles patent 358,525 from 1887.]
The first missing link Paul Wirt feed is an over/underfeed. Externally, it appears to be another Wirt overfeed. However, underneath the nib (but completely inside the section) are two pieces that together closely resemble a typical underfeed with the external end cut off. It is not fully round, and is a bit flattened on top. The nib sits on the internal grooved underfeed, and then a flat overfeed rests on top of the nib. Unlike the earlier completely flat overfeeds, this one has a shallow channel down the underside against the nib. This appears to be the feed in Patent 706141 August 5, 1902. And it is also interesting that this is the feed on my two line imprint pen (pre 1903) with the “transitional cap” as well as on the spiral case pen with the wide middle piece, the body style of which appears in the c1903 catalog.
One part of the underfeed was firmly in place. The second part is seen in the upper right of the picture.
Next is the underfeed as seen on patent 724,984 April 7, 1903. You might be thinking that this is simply the Wirt underfeed patent, since it is imprinted on all the underfeed pens. But there is an interesting thing most people do not realize. Although Wirt pens with underfeeds cite the April 1903 patent, the traditional vented underfeed as it was actually produced does not appear to have been patented until much later. Patent 978,420 December 13, 1910 is clearly the production model of the feed used from 1903 until nearly the end of Wirt production in the 1920s. The April 1903 patent cites two pieces to the feed. Within the barrel is a solid piece on the bottom, and the underfeed itself is flattened and rests on top of it. It is interesting that this feed also appears on a two line imprint pen with the transitional cap design. Just as the cap style transitions from a straight cap into a cone cap, with a brief layover at the transitional cap, we also see two transitional feed styles on this transitional style pen. This is also the feed on my double reservoir pen (patent 651738 June 12, 1900). It seems that Paul Wirt was taking rapid evolutionary steps in feed design in the final year or two before the major transition to the three line imprint, vented underfeed, and cone cap.
flat underfeed vs typical underfeed 
flat underfeed removed showing second piece of feed
