I have no more ideas about this car; zundapp seems to have built only the Porsche project in the '30, and this is not that streamline beginner, while Nsu-Fiats were very similar to Fiats, so, unless this is a coachbuilt one, I don't think we have a Nsu-Fiat. Then, we have been told that the maker, known for other products, built a well known car only, together with some projects...and this doesn't fit with the makers you proposed.
It's a real puzzle.