If you google
"1937 – Der erste Dethleffs in Rostock"
you will come across the German language story of the early years of Dethleffs caravans.
There you find a photo of a certain Mr. Franz Balge from Rostock/Germany, who drove this special convertible in 1937.
The name of the photo file suggests that the creator of that website thinks it might be a converted Adler or Auto Union,
which I highly doubt, to be honest.
I think the basis car could be Italian, most probably a Fiat?
I would love to find out more and to help identify this here
including the name of the coachbuilding company.
Some more pictures from that story: