No luck (Glück)!
I think I call this puzzle solved as I am no longer sure that the name I have really was the one of the company responsible for the conversion. So here is your point for identifying the base car.
The picture is one out of a set of three, one of them with a name and address typed on the back. The seller stated that it was the coachbuilder. I couldn't find any trace of it however. I even went through an address book of Merseburg from 1940 and there was neither a company with that name, address or telephone number, nor a citizen with that name. That doesn't exclude the possibility entirely, but I regard the typed name not as enough of a proof anymore.