[Arjay1949]

Further to Graham's information on the tail fins, it seems to get curiouser:

If JP137 was a MK II series 1, then we should expect Merlin XX engines, if my understanding is correct?

If JP 137 was a MK II series 1A, then I think we should expect Merlin XXII engines - and modified tail fins, as he suggests.

From the record cards, the reference to the tail fins not being modified suggests a MK II series 1.
The fact that the other record card clearly describes Merlin XXII engines, suggest a MK II series 1A ?

Unless of course these designations were not 'absolute', and there were examples of aircraft with a mixture of various modifications and engine types?