It's the seat base. A GRP box will be laid up into the space and filled with PU foam, traditional racing car style, once the rider has been bolted into it, Result is a perfectly fitting seat - for that rider. It could be you! (If you can get your bum into a 17" slot..)

If it wasn't for one of the top wishbone pick-ups, just visible top right, it could be a 1969 F.3 car, although the seat's a bit high for that. Definitely uder 20" at ride though (or indeed the 18" mentioned elswhere) so it's almost certainly an FF.