This odd period style accessory is a "four-seater" conversion introduced in 1957 by Scottish MG Distributors John Gibson & Sons Ltd. of Edinburgh. Apparently the spare tire goes on the boot lid. I have no idea where the passengers' feet go, unless maybe a substantial part of the rear bulkhead panel is removed. And if you're going to put two people in the boot of the MGA, they would surely neeed to be very friendly (or very small). Could be a wild ride though.