Piston-fillers, exemplified by modern Pelikans and Montblancs, have occupied a prominent place in European pen production since the 1930s. American penmakers, however, avoided the piston-filler, the Conklin Nozac ("no sack") being virtually the sole exception. The Nozac was made for a relatively short period, from 1931 to around 1938. Production numbers were not great; these were Depression years, and Conklin had been losing ground for years to giants such as Parker, Sheaffer, and Wahl-Eversharp. By the time the Nozac was introduced in 1931, Conklin could no longer afford to promote it in an increasingly competitive marketplace.
Despite this, Nozacs came in a great profusion of sizes, styles, and colors. Some are round, others are either partially or fully polygonal, with either 10 or 12 sides. Markings may include "Nozac", "Endura", and "Symetrik", singly or in various combinations, or there may only be the Conklin imprint with patent information. Many pens bear a "Word Gauge" numbered gradations on the transparent barrel indicating how many words could be written with the remaining ink (standard-sized pens went up to 5000; oversize, to 7000). Matching pencils were offered, as well as desk sets.
Nozacs are not the easiest pens to service. This is partly due to American repairmens relative lack of experience with piston-filler seal renewal (some still use thick disks of stiff rubber or other equally unsuitable materials; the original seals were alternating layers of cork, rubber, and cork, as visible below), but it must also be noted that the Nozac mechanism itself -- the hollow aluminum piston shaft, in particular -- is rather delicate and prone to binding and deformation. If you find a pen with a stuck piston, do not force the turning knob, as this can cause irreparable damage. Nonetheless, repair is generally straightforward as long as the piston operates properly. Easy-to-install replacement seals are listed here.
The illustration above shows the mechanism used with a two-part barrel assembly. Most Nozacs had a one-piece barrel, using the same piston assembly design. A scarce variation used a beefier hexagonal piston shaft.