Vintage Cars: 1965 ford mustang, ford mustang convertible, mustang engineering

Question
what explanation can you give me when a 1965 ford mustang convertible has a vin code 7 which indicates that its not a convertible.

Answer
Well....how do we explain that only a certain number of very high-performance 1967 Corvettes were originally built, yet there are now MORE than that number in existence?

Because forgery is a very worthwhile business.  Wouldn't a 1965 Mustang convertible be worth a lot more than a '65 Mustang hardtop coupe?  How hard would it be to take an old coupe in need of restoration (one that would be quite cheap to purchase), cut off the roof, install the convertible top mechanism and sell it as a true convertible?  I'm speculating here, but it's the only explanation I can come up with.  I'm not totally verse in Mustang engineering and structure, but I know of no convertible within a lineup that did not have a stronger frame than its corresponding coupe model.  Have an expert check out this car to determine if it has a true convertible (ie, stronger and beefier) frame.