We are both right, it's just that the sources are contradictory. As you stated Regenberg (as well as Michels in his Borgward trucks and buses book) state that it is a Borgward B 3000 chassis. The picture source, a 1949 magazine calls it an Opel Blitz chassis, but also states in the text that this body was made for chassis from various manufacturers. So I guess they used the Borgward picture to illustrate the body, especially as it wears no visible identification of the chassis brand. One more point for you.
