Thanks Ramon..I'm not quite as familiar with some of the details of the 'early' M20s as I've never owned one, so it's just a question of whether I can remember what I've read or observed......Ian ...
If you look at visitors bikes you will see my M20 third from last posting. The filler cap on this is three and a quarter inches dia.
The oil filler cap is two and a half inches dia, I am fairly certain both are original to the bike
My tank has two outlets one is blocked off operates of a single pipe only this appears to have occurred a long time ago. This M20 was released by the MOD on the 31st July 1970 and stored mostly thereafter.