A figure of 4517fpm initial climbrate is also quoted on J.Baughers
website , which gives the following sources:
Japanese Aircraft of the Pacific War, Rene J. Francillon, Naval Institute Press, 1979.
Famous Fighters of the Second World War, William Green, Doubleday, 1967.
Zero Fighter, Martin Caidin, Ballantine, 1970.
War Planes of the Second World War, Fighters, Volume 3, William Green, Doubleday 1964.
The Mitsubishi A6M2 Zero-Sen, Rene J. Francillon, Aircraft in Profile, Doubleday, 1969.
Mitsubishi A6M5 to A6M8 Zero-Sen, M.C. Richards and Donald S. Smith, Aircraft in Profile, Doubleday, 1974.
I would guess that this (suspect)climbrate figure originally came from one of Francillons books?