A classified World War II report on the breaking of German secret service codes was presented to the only surviving member of four codebreakers who wrote it.
Bletchley Park codebreaker Mavis Batey was presented with the newly-declassified History of Adwehr Codebreaking 66 years after she helped decode it.
Ms Batey, now 90, was told the document, dubbed Batey, Batey, Rock and Twinn after its four authors, including her husband Keith Batey and colleagues Margaret Rock and Peter Twinn, would never be declassified.
But, in an historical moment, intelligence agency GCHQ presented it to the Bletchley Park Trust and Mrs Batey, the only survivor of the four codebreakers.
She travelled from her home in Petworth, West Sussex to Bletchley Park and saw for the first time what her codebreaker husband had been working on - the couple each wrote independently for the report and never gave the secret away to each other despite being married.
The document was one of several histories written by each Bletchley Park section at the end of the war.
Reports from hut eight and hut six, covering German naval, army and airforce codebreaking, have already been released but the report relating to the German secret service, the equivalent of MI6, remained classified.
Despite releasing hundreds of thousands of World War II documents, GCHQ has only released one other technical history – The General Report on Tunny’, a Bletchley Park spokesperson said.
The Batey, Batey, Rock and Twinn report covers the work of Intelligence Services Knox at Bletchley Park, whose head until his death in February 1943 was Dilly Knox. German literature student Ms Batey, nee Lever, who has since written a biography about Mr Knox, was not quite 19 when she arrived at Bletchley Park, Buckinghamshire, in May 1940.
Later entrants to the Government Code and Cypher School (GCCS) - which later became GCHQ - undertook a six-month course at the Inter-Services Special Intelligence School at Bedford before trying to solve machine ciphers.
But she was thrown in at the deep end, greeted by Mr Knox with the words: “Hallo, we’re breaking machines. Have you got a pencil? Here, have a go”.
He handed her some sheets of code, and despite apparently finding it “all Greek”, she went on to become a key member of the ISK team. Ms Batey, who married fellow codebreaker Keith Batey, was one of only about three skilled female cryptanalysts at Bletchley, together with Margaret Rock in ISK, and Joan Clarke in hut 8.