(_Geschichte des Automobils_, Richard v. Frankenberg & Marco Matteucci)
So: Yes, Ferry Porsche (the son), with chief designer Rabe, designed a Cisitalia GP in ~1946/-47.
In 1947/-48, the great (once -- way past his prime by then) Stuck drove the Cisitalia 1100 in several races; won one, got one second place. Also in 1947, the Great -- still, at 54! -- Nuvolari places second in the Mille Miglia, also in a 1100... After having led for most of the race, only to have rainfall cause ignition problems that he lost 15 minutes repairing.
Dunno for sure if this Cisitalia 1100 is the car Ferry Porsche and this Herr Rabe had designed a year before, but it seems not *too* unplausible...
Surprise: Yes, the 1947 street car with the Pininfarina body certainly looks as if it has a back seat, on the photo in my book! :-)