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 until c. 1930 when the venerable design was finally retired in favor of the lever-filler, adopted by Conklin several years before.
Crescent-fillers with fancy metal overlays are prized by collectors, but ordinary specimens are still available at quite reasonable cost. All are top-quality pens, and many carry flexible nibs. Beware of pens with missing or damaged lock rings, however replacements can be very difficult 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. Slip caps no longer appear in advertisements or catalogs after 1913; screw-caps were introduced in 1909. Mottled and red hard rubber crescent-fillers are quite scarce and desirable. Early pens made up to c. 1907 had unmarked crescents (the crescent for the all-metal "hog-ring" model is an exception); pens made up to c. 1920 had the crescents marked "CRESCENT-FILLER/TRADE MARK" on one side only, while pens made from the early 1920s on had crescents marked on both sides. Narrow feeds are an early feature, replaced c. 1913 by a wide, flat feed with notches cut on either side.
For more information, see The Conklin Legacy (2013) by Alfonso Mur Bohigas.