One of the great pioneer penmakers, Conklin’s heyday spanned the first third of the 20th century. Roy Conklin’s 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.