SUDAN ↔ UGANDA airmail cover to Butiaba from first regular mail service Cairo to Kisumu in Kenya by Imperial Airways (as first sector of later London/Cairo to Cape Town route) that opened in March 1931.  Wadi-Halfa special datestamp SUDAN AIR MAIL of 12 Mar 1931 on front with Kisumu 15 Mar 1931 and Kampala 18 Mar 1931 transit datestamps at back