Well, I found traces of the Magirus Saturn III. I assume that it was introduced after the model changes in 1963. One is the Cologne Saturn III MS from 1964 (see post #30) and there is also a workshop manual for Saturn I, II and III from 1963, but I only have a type designation for the puzzle bus according to the scheme introduced in 1964. It should therefore have been built between 1964 and 1966. A small step in the roof was introduced in 1966, which it lacks. So this puzzle is still unsolved.