Hi Toro,
You may have a very rare car. American Sunroof Corp, now know as American Specialty Car produced all of the GM come back convertibles and most of the Chryslers of the 80s and 90s. I also have a 1948 Pontiac 2000 STE convertible by ASC. They did convert the E body Cadillac Eldorado and Buick Riviera but don’t believe they did any Toronados. I have seen them around but only the ones build by Coach Builders and several hearse fabricators, one being Scoville. If the original title classified it as a convertible it’s probably authorized by GM making it quite rare. If its states coupe it’s just a regular conversion.
http://www.ascglobal.com/ here is a link explaining what ASC now does. They might be able to help you out.
Good luck
Chesler