Unless you have a SPEC-FO number on the body tag, or the 999/888 paint and trim numbers, this "factory prototype" could very likely be someone's attempt to make a convertible after the fact. A factory built car should also have a notable number of ADDITIONAL body mounting points to help compensate for the loss of rigidity that occurs when the top was cut off. They found this out when they did the 1951 prototypes in the Experimental Engineering department.
Jack Mueller