Zeiss released both of their long Contax lenses in 1934. The 300/8 was a very sharp telephoto, but of small maximum aperture. It was replaced in 1940 by the 300/4.
It was designed to be used mounted on a tripod. The lens would be focused on a ground glass attachment. After focusing, the ground glass would be replaced by the camera. This was a cumbersome arraignment, which resulted in a very limited production, estimated at about 200 lenses. The lens barrel was modified for the introduction of the Contax III in 1936. The shoe height was increased so that the finder would clear the meter on the Contax III.
When the Flektaskop was released in 1938, a few of these lenses were remounted to fit.
The 300 mounted on a Contax I, with the special yellow filter attached.
A close-up of the lens lock lever.