The book USAAF Camouflage of World War 2 by Jerry Scutts states "A further marking of MTO US aircraft was the RAF fin flash, used as an Allied theatre identification marking following
an order to that effect in Dec 1942. There are photos of P-40's and B-24's with US markings and the red-white-blue fin flash around. A lot of the Ploesti B-24 bombers also had the
yellow outer circle around the US blue and white star and some with the bars added The thin yellow outer circle was also a MTO theater marking