There is so much scepticism in the market and on ebay about 'manufactured' errors, especially after that 20p farago, that probably the only way to go is to send it off to CGS for an opinion as to whether it has been tampered with or is a genuine mistrike. It'll cost you a tenner, they will probably say it's been tampered with, but you never know and if they certify it and slab it, then who knows what it might fetch, no-one can say. In the meantime, if it is genuine, then we need to find some more as there will be some. You'll be helped greatly if someone finds another one.