Finlay Shaw was a Cadet prior to being promoted to 2nd Lt. Royal Flying Corps, 1 OCT 1917. Supplement to London Gazette, page 11024, 26 October 1917. On 1 APR 1918. Finlay Shaw and all of the others in the RFC were transferred to the new RAF. However for a short time after the formation of the RAF, pre-RAF ranks such as Lieutenant, Captain and Major continued to exist, a practice which officially ended on 15 September 1919. Wikipedia. This explains how he was a 2nd Lt. when he left the RAF even though that was not officially a RAF rank.