In a book on Duxford "Duxford and the Big Wings 1940-45," by Martin Bowman, 2nd. Lt. Dick Hewitt states that, at the end of 1943, his P-47 was fitted with IFF. By then, the IFF Mk.III was in use, and (on RAF aircraft, at least,) the only visible sign was a bar aerial, usually under the wing or fuselage, about 12" long.