This o ring has a number of applications:
1. It is the smaller inner o ring for the rear shackle pin in the MkVI and R Type range (2 per spring)
2. It is the o ring on the engine breather system of later Series 2 and all Series 3 S Types and Silver Clouds and Phantom V
3. It is the o ring inside the early Shadow and T1 4 speed gearbox between the intake pipe and the front pump

1945-1955 MkVI & R-type 

1956-1965 Series 2, all Series 3 S types & Silver Cloud 

1966-1980  Early Shadow & T1