I think the previously puzzled Opel wasn't made by Tröller. Simsa refers in his book just to Tröller as one of those coachbuilders offering rebodies for old chassis. So this is probably the first real Tröller body here.
Now for the base of this one: you said that Adler was too easy, so it must be a more obscure base car, at least in Germany. Is it a German base car at all?