Possibly mentioned on here before, but here's a link to what claims to be the 1st Cabriolet and it was built on a VW chassis.
http://tinyurl.com/4tl2v2
1948 Porsche Prototype Custom Sports Cabriolet Chassis #: 1-085552 - Engine #: 115914 1949 Geneva Show Car! This car was built in Zurich by Hans Waibel, who in 1988 declared that he had show prepared two Porsche coupes at the request of Ferry Porsche and Louise Piech for display at the Vienna Industrial Exhibition. Waibel was approached by a client, Rupprecht von Senger and asked to build a similar car, which was created taking a VW chassis 27cccm longer than the Porsche 356 chassis that arrived in Switzerland on October 18, 1948, removing the body and fitting a Porsche light alloy cabriolet body and Porsche engine. The tunnel backbone of the VW platform was fitted with an electric fan which channeled air from, a Morris Minor grille to cool the hot-running Porsche engine this being the first Porsche
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