Very late 5TA - or not?

Hi - just dug out a project I've had sitting around for far too long but before doing anything thought I would check the numbers out but cannot make much progress. Bike has no docs. I have lots of literature on what the frame and engine numbers should be but the engine is cleanly stamped 5TA H57xxx and doesn't look like it has been messed with and I cannot see any stamps on the frame which looks like a late genuine frame. The books all say that the 5TA engine numbers stopped well before that number with the last one being H46431. H57xxx makes it 1968 but 5TA ceased production in 1966? Can anyone shed any light on what I may have? Ta very much