21. Training manual. 2170-6. Air Corps: Aerial photography, prepared under direction of chief of Air Corps; Jan. 2, 1930. Bookmark Corporate/Agency Authors: War Department Publisher: War Department Date: [1930.] SuDoc number: W 1.23:2170-6
22. Training manual. 2170-35. Air Corps: Identification of aircraft, prepared under direction of chief of Air Corps; Nov. 18, 1929. Bookmark Corporate/Agency Authors: War Department Publisher: War Department Date: [1929.] SuDoc number: W 1.23:2170-36