This is the clear park lamp and also known as the reverse lamp on some models.  You will need to provide the car's VIN/chassis number to make sure this will fit. 1946-1965 Various Models -...

Gasket to the Reverse Lamp Lens to the Body of Car 1959-1965 Cloud II,III & S-2,3 1959-1968 Phantom V

This is the Tail Lamp Body to Lens Gasket. 1962-1965 Cloud III & S3 1968-1980 Corniche, Shadow, T, Shadow II, T (from VIN 5001-41573)