These three Schildkrote photos are a hoax (a joke), not of a real disc but of a model made for fun. You'll find them all over the internet, but they are NOT of a real craft!
Source: "Luftfahrt international" issue # 9
Publisher: Publizistisches Archiv Karl R. Pawlas
Nürnberg (Nuremberg), w/o year (mid-1970s ?)
no ISBN known