Hey there Guest!
The back section is a upholstered hard board that covers the gas tank, as Randall said. That item was used on all cars. The seat bottom is a plywood with foam and upholstery. The bottom "seat" was an option. I don't have the catalog handy but I'm sure Moss, Victoria British and Roadster Factory have them. You would have to buy the hard board, which is shaped to fit and I think the upholstery part is sold separately. Another source is 2tall custom interiors which post a lot on ebay for interior sets and seat, but would provide individual pieces. I occasionally see the back sections on ebay too.
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Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
