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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. 5.
Art. 5. Non-military public aircraft other than those assigned to customs or police service must, in time of war, carry the same exterior marks and shall be treated, as regards the present rules, in the same manner as private aircraft.