He repeated that the three digits were in fact his full serial number as he had previously served in the First World War as well
I have a vague recollection of reading that in John Bushby's book "Gunner's Moon" - I last read it a couple of decades ago, so I may be wrong.
It was pointed out on Twitter recently that one of the WAAFs who were awarded Military Medals at Biggin Hill during the Battle of Britain also had FWW medal ribbons, having rejoined when the WAAF was formed.