I think these cassettes are made for reproduction work with the Durst Otoka-system. I own some cassettes that look exactly the same, with different inside formats (6x9, 9x12 and 13x18) for the Larka-system that fits on the Durst G139 enlarger. Durst had different systems for reproduction with enlargers (Ursix, Urnov, Otoka, Larka and Larka 205 - according to an old copying manual), all with different sizes of cassettes.