Let's be clear about the fact that the prior thread contains photos of two separate cars. The black Köng/Dietrich conversion (subject of the prior puzzle) and the ivory "taxi" are not the same car. In fact, these cars have even appeared together at Borgward meetings.
Is this puzzle car the "taxi" from the other thread? If so, the extent of what I know is that it has been called a "homemade" conversion.