5M is the Squadron code for the 15th Tactical Reconnaissance Squadron.
https://www.afhra.af.mil/About-Us/Fact- ... adron-acc/Chilbolton, England, 1 Mar 1944; Middle Wallop, England, 16 Mar 1944
https://en.wikipedia.org/wiki/15th_Attack_SquadronDuring the early stages of World War II, the 15th supported the Field Artillery School in Oklahoma. On 26 March 1944, the unit deployed to England and began combat operations over France. Its first combat mission was photographic reconnaissance on a North American F-6 Mustang. On 6 June 1944, the 15th received credit for the first aerial victory by a tactical reconnaissance pilot as well as the first victory of D-Day. The unit continued armed reconnaissance operations in the European theater until July 1945. After returning to the United States, the squadron provided visual and photographic reconnaissance and artillery adjustments for Army, Navy and Air Forces until it was inactivated in April 1949.