What is meant by Internal or External Tank?
The vast majority of Traveller and Countryman cars have are external tank.  The fuel tank is located underneath the rear floor of the car with the filler cap low down on the right hand side of the car.

The very early Traveller and Countryman cars built before October 1961 had what is termed an internal tank.  On these cars the fuel tank is inside the load area on the left hand side and the filler cap is located just below the rear windows on the left hand side of the car.

Internal Tank

External Tank