Warmtoast (#14),
I'm surprised that John would be wearing "mufti" in wartime, to my recollection we wore uniform all the time during the war. Might be wrong.
The "white flash" was issued to all airmen under training for aircrew trades (or was it just pilots and navs ?) I was told it signified "Potential Officer" (analagous to a midshipman's white patches or the white Cranwellian capband. But this has been disputed by other contributors here.
It was a small wedge of felt-like white material, and fitted in the front fold of the Cap, Field Service ("Side Hat" or - whisper it - "C**t Cap" - from the obvious anatomical resemblace.
Danny.