This is all I have:
This 'thing' was advertised for sale in an Italian magazine in 1977 in 3 different issues.
1st it was the "Autonave Mercedes Prototipo" the longest convertible in the world, wooden prototype, 10 meters long with a pool.
Then it was a "Prototipo Autonave" with a pool on the rear, wooden model, longest cabriolet in the world (10 meters) under the Mercedes ads and finally it was simply "Carlangelo", wooden prototype, extraordinary (10 meters), to be fixed, Fiat 1600 engine included.
The price started from 1.300 lire and ended at 1.800. Seller was always Mr. Carlangelo in Poggio Mirteto.