Mysteriously appearing at the U.K. Embassy in Oslo, Norway, in the very early days of World War II, the Oslo Report was an annotated collection of documents on Nazi weapons technology and military techniques. The anonymous author asked for a minor change in the BBC broadcast if the British wished to be apprised of German scientific developments.
After the broadcast was sent, the document was received on the night of November 1-2, 1939. R.V. Jones, the head of (and for some time the body of) the scientific intelligence section of the Secret Intelligence Service (SIS) reviewed it, and at first it might be deception, as the material in it dealt with extremely critical items. Most, however, proved to be completely accurate; those that did not were information overheard by the source.
SIS never learned the identity of the source during the war. Individuals with access to the information, however, considered it an extremely critical one. Only after the war did Jones meet the author, in the mid-fifties, who had never been detected by German counterintelligence. Jones agreed not to reveal the source's identity until the source and the source's spouse both had died.
Jones revealed, in the 1980s, that the source had been Hans Ferdinand Mayer, who had been the director of the Siemens & Halske research laboratory in Berlin.