The following is an excerpt and was found in NEW YORK KING'S COUNTY CLERKS'S COURT RECORD 1940 vol. II P. 795. It has to do with court testimony that was being made by Fred Franz in a lawsuit brought against the WTBTS by others.
"LAWYER: Who subsequently became the Editor of the magazine, the main editor of the 'Watch Tower' magazine? F.FRANZ: In 1931, October 15th, as I recall, the 'Watch Tower' discontinued publishing the names of any editorial committee on the second page. THE COURT: He asked you who became the editor. F.FRANZ: And it said... THE COURT: Who became the editor? LAWYER: Who became the editor when this was discontinued? F.FRANZ: Jehovah God.".
Note: F.Franz later became the President of the Watchtower Bible and Tract Society. Based on his testimony, Jehovah God is the editor of the Watchtower since 1931. Therefore, since this date the magazine could not contain any errors since God is its editor.
Here is another one.
This is an excerpt and was found in an Action for Libel: Olin R. Moyle v. F.W.Franz, et al, May 10 - May 27 1943, New York Supreme Court, Appellate Division, sections 2596-7, 4420-1. It has to do with testimony that both Fred Franz and Nat Knorr made in this case about who is the Editor of the Watchtower.
"LAWYER: At any rate, Jehovah God is now the editor of the paper is that right? F.FRANZ: He is today the editor of the paper. LAWYER: How long has he been editor of the paper? F.FRANZ: Since its inception he has been guiding it. LAWYER: Even before 1931? F.FRANZ: Yes, sir. .. LAWYER: But you don't make any mention in the fore part of your Watch Tower that 'We are not infallible and subject to correction and may make mistakes'? N.H.KNORR: We have never claimed infallibility. LAWYER: But you don't make any such statement, that you are subject to correction, in your Watch Tower papers do you? N.H.KNORR: Not that I recall. LAWYER: In fact, it is set forth directly as God's Word, isn't it? N.H.KNORR: Yes, as His Word. LAWYER: Without any qualification whatsoever? N.H.KNORR: That is right."
Note: N.H.Knorr was, and F.Franz later became, the President of the Watchtower Bible and Tract Society. Based on this testimony, Jehovah God is the editor of the Watchtower. Therefore, the magazine could not contain any errors since God is its editor.