States that the 1995 was "the only year for the convertible hardtop, and with only 499 models available, it has become a sought after collectible. The hardtop/ragtop was installed as a prototype on a few V6 and GT Mustangs, which have found their way into private hands, but were never "officially" produced. " With about 2 dozen photos, and http://www.themustangnews.com/gallery/showimage.php?i=92&c=12 has some more
In 1966 a Ford designer who was the man behind the retractable hardtop '57 - 59 Starliner was tasked by Ford to get the 67 Mustangs a hardtop convertible option package, which didn't happen in production due to the weight of the motors etc to retract the top.
What made me look into this was that Tere had a copy of Mustang Monthly that had a 1966 "Towne Top" detachable hardtop. http://www.mustangmonthly.com/featuredvehicles/mump_0801_1966_convertible_towne_ragtop/index.html
Dory Development Corporation of Salem, New York made these limited-production hardtops for Mustang and Corvair convertibles, about 200 of them, and 3 are known to still exist.