The photo of pnegyesi can be found in two books. In both cases, Dieter Malkämper is named as the source of the photo and the basis is a Borgward B 1250. The customer is said to have been the bus operator Büsch.
And Borgward does not rule out Opel as the basis. For reasons of economic production, it was not unusual to mount the same design and the same body on different chassis. In this case, it was probably a good fit, because the wheel arches, for example, were very similar. Borgward B 1250: 3200 mm, Opel Blitz 1.5 t: 3250 mm.