A Ferrari 400 GT manual is rare. A Ferrari 400 cabrio is rare. But a Ferrari 400 GT manual Cabrio by Stommel is unique. I came across these pictures so adding them for the records. Small things I noticed, it has a Ford branded radio (Yuck), the missing chrome trim on the front bumper missing, the rear lights missing the chrome rings, incorrect side mirrors. The written Pininfarina bage moved to the backside?
As for the company itself I couldn't find much more about them. The only thing close I could find is this company called Stommel and also located in Koln and active in the automotive business. But it's not revealling anything on the company history: http://www.identica-stommel.de/de_DE.html