To meet the US design laws, yes they do actually have them, the original Californian Moke that was destined for the US Virgin islands was fitted a rear fuel tank as side fuel tanks were no longer allowed. The rear tank was squeezed up inside the subframe and mounted to the floor with a filler in the rear panel and a neck going through the rear tray area behind the seats. The tank was mounted offset to make room for the exhaust to come down the left hand side.

Not shown in the pictures is a block of foam the was packed around the filler on the tank and pressed up against the underside of the rear floor with the intention of keeping the dust out of the back of the Moke, the foam acted like a sponge and absorbed the dust and water and caused many tanks to rust out around the neck on the tank.