Perhaps this helps (there is not only the type of material but also some advices for cleaning):

http://www.talkmorgan.com/ubbthreads.php/topics/499188/re-sidescreen-misting#Post499188

However, the information concerns the material that is currently being used. It was certainly not the same 37 years ago.
In any case, if you want to replace the screens, I would use the current one.

Michael
2018 Plus 4 Sport