At the RAF Museum, Hendon, London. Amazing place. There were *loads* of markings used by the RAF which makes me wonder how on earth they remembered them all and crucially, distinguished them apart from enemy markings, especially at great distances and in poor visibility.
The Night Flying Roundel [top middle] 1916-1918 is a good exmaple of a marking used but being pretty useless. "As both bombers and fighters began to operate at night a white ring was used to assist in identification in poor light. Rarely used."