Band on the Run,
The letter can be found in the Nazi archives. The Soviet Union and the GDR (East Germany) used the letter against the witness. Also the West German press published the letter in the '60 or '70. Also the Watchtower Yearbook of 1934 (english version) containes the text of the declaration of facts.
Clear is that the letter is written by Rutherford & Knor during their Germany visit.