Given that ARP appears on all sorts of items, it appears infrequently on helmets. The below image is one of the few I have seen that clearly shows ARP on a helmet. The actaul wording to this (most likely staged) 1941 photo is:
"The good neighbour. Besides their function as the eyes and ears of the Control Centre in the field the wardens had another, equally important, that of the 'good neighbour' in the blitz, earning and keeping the people's confidence. In this role the work of the women wardens was outstanding. One warden in every six was a woman."
News about interesting insignia, ARP related info and period photos that turn up.