Brian-
Welcome to the Stovebolt. I'm not an interior historian, but others will be able to add researched responses. Lots of help here.
I'm rebuilding a '51 and the seat I pulled had a thick synthetic material like vinyl that may have been aftermarket, but I think it was original or from the same era. Don't count vinyl out just yet.
One option is to look at parts distributors listed at the Stovebolt
Link for Interior Parts for a supplier that can help with materials and info.
Cheers-
