I believe Stevens roofs are interchangeable. The only plexi roof I am aware of was made by Saratoga during the GM years. It is possible that the 03 was using one of these. Unlike the later glass tops they do not require seal replacement as the roof comes with its own sub frame and seal that rides in the opening gutter. Later model cars have glass roofs. It would be easy to tell via pictures as Plexi roofs do not have any fittings on the exterior for a total smooth look. The glass roofs have low profile top side fasteners as the glass is drilled for the hardware. I sold my plexi roof to Dave Cammack's X180R when I sold my previous 90 SE
Here's a photo album of the latches and glass top on my 88 Stevens Esprit - the top reportedly came from a V8 (of unknown year).
I put gasket material on the glass top to work with my existing composite roof gasket.
But I do have the 'glass top' gasket that now I hear actually works fine with the composite top. Will try it after I finish some other interior mods.