From 1937 – 1942, Wilhelm Albert designed paired lenses, held in a common focusing mount for stereo-photography. These lenses produced a pair of 18 x 24 mm images within the full 24 x 36 mm frame. A single ring controlled the diaphragm of both lenses.
Two versions of such lenses were made during WWII, both of 3.5 cm focal length and with the typical black finished front plate: the f/2.5 Stemar and the f/3.5 Stereo Elmar. In the 1950s, this design was further developed and turned into the familiar 3.3 cm Stemar lens system.
The wartime stereo lenses are exceedingly rare, as less than 50 units were produced. Usually, the lens mount was equipped with a receptacle for holding the arm pin of a specially modified VIDOM viewfinder.
Here is a most unusual prototype Stereo Elmar lens is being offered which lacks such a receptacle for the finder. This does not represent a later modification, but its original status. Presumably the lens was intended for experimental use in combination with a different finder solution.
Literature: Wilhelm Albert, Die Geburtstage der Leitz Fotokonstruktionen seit 1927, Wetzlar 1990, p. 49-56; James L. Lager (ed.), Leica Illustrated History, Vol. III, 1998, p. 277-278; Paul-Henry van Hasbroeck (ed.), Leica, a history illustrating every model and accessory 2nd. ed., colour plate XXIV.