I found this picture on the net, but the caption doesn't mention the coachbuilder. I tried to obtain a clue from Rohr Junior history; this car was a Tatra T75 built under licence by the Neue Rohr Gmbh, and I found that F.Porsche was involved in the project. The car (as Bill said) looks like Mercedes-Benz H, so I'm convinced that the coachbuilder is neither Czech, nor Austrian. As it is quite streamlined I will guess Ludewig.