In a message from the Universal House of Justice to a department at the World Centre, Amatu’l-Baha Ruhiyyih Khanum is quoted as saying:

…that the English dictionary to which the beloved Guardian habitually referred was “Webster’s New International Dictionary of the English Language”, second edition, unabridged, 1934, London G. Bell and Sons Ltd., Springfield, Massachusetts, R. and C. Merriam Co. We presume that if there were two versions published, one American and one British, it will have been the British one that the Guardian used. Reference to this specific edition of this dictionary is, obviously, very important when gauging the exact meaning intended by Shoghi Effendi in the use of certain words. [Dictionary Used by the Guardian by/on behalf of Universal House of Justice 1997-12-08]