I know of a guy who's got one and he claims its a rs turbo cabriolet on the log book, he said 3 were made, left the ford factory
They made the SE500 which is an XR3i Cabriolet with an RS Turbo bodykit, but never an RS Turbo engined cabriolet.
I would highly doubt it says "RS Turbo" and "Cabriolet" on the log book unless it's a Q plate.
Princess Di had a black S1, non cabriolet. She may have had another Cab
Lee