A 1939 circular from the Home Office decreed that the letters on a wardens helmet were to be two inches in height. In London the W was to appear both on the front and rear of the helmet. How this was interpreted by various groups varied markedly and various shapes and fonts of the W can be found.
On this photo, the warden third from the right clearly had a different size ruler to everyone else...
News about interesting insignia, ARP related info and period photos that turn up.