It is a 540-based early modified from Mick Goddard, initialed by him and also designated 'CBB' spec. Presumably he has opened it up, ballraced it and rewound it as a 27T single.
It has taken a little digging to date it and work out what that means, but I have come up with an old MG advert from April 1983 which actually advertises a 'CBB spec Modified' in 05 & 540 guise.
This motor has a really great label condition and I have never been so chuffed to get my hands on a 540 silver can before!
