Are you saying that you bought the bike on the basis that the engine and frame number matched but you have now found that the engine number is different and relates to post-1969? Or are you saying the numbers still match but there is some feature you can see on the engine cases which makes them post-1969?
Either way, if you contact the club's National Secretary and tell him what the frame number and engine number is, he'll be able to tell you when it left the factory and in what form (hopefully, as an A65T).