One of the great pioneer penmakers, Conklins heyday spanned the first third of the 20th century. Roy Conklins great innovation was his distinctive crescent-filler, the first mass-produced self-filling pen as well as the first mass-produced pen to use a flexible rubber ink sac. The essential patents were granted in 1901 and 1903, but crescent-filler production continued into the mid-1920s when the venerable design was finally retired in favor of the lever-filler, adopted by Conklin a few years before.
Crescent-fillers with fancy metal overlays are prized by collectors, but ordinary specimens are still available at quite modest cost. All are top-quality pens, and many carry flexible nibs. Beware of pens with missing lock rings, however replacements are not easy to find. This is further complicated by the great variety of measurements found among crescent-fillers bearing identical model numbers. It is not uncommon to see a Conklin #5 nib of smaller dimensions than a Conklin #3!
Most crescent-fillers that you will find have screw-caps, but early models used a slip cap. Mottled and red hard rubber crescent-fillers are quite scarce and desirable.
For more information, see the articles in Pen World, vols. 7/1 and 7/2 (1983).