Introduced at Photokina Cologne in 1954, the Retina IIIc was the most expensive of the new range of Retina models. Compared to the earlier models (1951 Ia & IIa), it was a complete redesign - the extending lens assembly now slid forward on the body and was thus much more rigid than earlier models. The front opening "door" no longer played any supporting role and merely moved the lens assembly in or out. The leverwind was moved to the base of the camera.
The IIIc mainly differed from its cheaper brethren by the addition of a f2 Heligon or f2 Xenon lens in place of the f2.8 variety found on the Retina IIc plus an uncoupled meter. The Retina IIIc with the Rodenstock Heligon lens was more commonly found in Europe and hardly ever in the USA, so it is surmised that exports to the States were limited to those brought back by returning servicemen. (Camerapedia)