From the manufacturer:
Mini Coopers from 1962 to 1973 had a factory option of fitting an extra fuel tank. This boot board will fit all these models as well as other saloons built before 1980 when a larger petrol tank was introduced. Made from plywood with carpet adhered to the board and a vinyl beading, which is stapled on the reverse side. The carpet color usually matches the interior carpet color.
This item fits over special brackets on original cars, but bracket kits are also available.
Special order, lead time required. Additional colors may be available; actual colors may vary from those shown.