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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 III: Belligerents. - Art. 15.
Art. 15. The crews of military aircraft shall bear a fixed distinctive emblem of such a nature as to be recognizable at a distance in the event of crews finding themselves separated from the aircraft.