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Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923.
Part II: Rules of Air Warfare#Chapter I: Domain of application: classification and marks. - Art. 8.
Art. 8. The exterior marks required by the regulations in force in each State shall be brought without delay to the knowledge of all the other Powers.
Modifications made in time of peace in the regulations requiring exterior marks shall be brought to the knowledge of all the other Powers before being put in force.
Modifications made in such regulations at the outbreak or in course of hostilities shall be brought by each Power to the knowledge of all the other Powers as soon as possible and at the latest when they are communicated to its fighting forces.