The code, written on paper headed "Pigeon Service", was discovered in a canister attached to the bird's skeleton, and found up a chimney at a house in Surrey.
Experts at UK intelligence agency GCHQ said the message is impossible to crack without its code book. They were also stumped by details such as the date of the message and the identities of the sender, "Sjt W Stot", and recipient, "X02".
A spokesman said about 50 people had come forward, from the UK, US and Holland, but no-one had a solution yet.
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