Royal Mail 'Water & Coast issue.
Artwork depicts World War 2 radar stations. A Zeppelin airship sent to investigate reported the strange 'towers' as radio masts. The message intercepted and decoded at Bletchley Park proved the Germans had no knowledge of a British radar system.
The full story is told in the cover insert. There is also a decoded copy of the message sent to the Head of Luftwaffe intelligence.
A lovely historic cover.