Because Zeiss had designed their camera with a removable back, they were able to offer their customers the option of using a back that supported small cut film holders, each holding one frame. They could be replaced with a ground glass for critical focus and framing. Although this was not a very practical, nor useful accessory, it was not only available for the Contax I, but was made available for the Contax II and III as well.
It did not sell well for either camera, and is not common today.
The plate back for Contax II and III
The same, with the dark slide pulled out
The back open, with the plate holder partially removed
The ground glass focusing insert can be used for critical focus.
The plate back removed from the camera, shown with a plateholder, ground glass and a box of two additional plateholders