FAA, RN or SEAC roundels, can't remember at the moment
The British Pacific Fleet aircraft used these markings, similar to U.S. markings in 1945, to reduce the friendly fire incidents by U.S. pilots and A.A. gunners unfamiliar with British aircraft(????)
The blue/white roundels were for all British aircraft assigned to the Pacific beginning in '43 (I forget which month exactly). They were to help reduce confusion, the same as the US North Africa insignia with the yellow/gold ring on the outside (matching the outer ring of the RAF roundel), by making the US and British insignia more similar for each theater. The standardized shape in each case also made it convenient to paint over insignia when needed to transfer aircraft from one service to the other.