An interesting item, not often seen in the UK, but this is an American Civil Defence helmet, based on the British Brodie helmet design (also used by the Americans pre M-1 helmet). Essentially, the American A.R.P. and although they never saw an air raid, they were similarly well organised as the British wardens.
The helmet is complete with rayon webbing suspension and chin strap, with press stud fixings, similar to the early war low fibre M-1 helmet liners. Has the blue circle, with red and white striped triangle, denoting use by an Air Raid Warden.
Seems to have M1 helmet style fixed bales rather than the rivetted bales common to Brodie style helmets. Liner is in nice order, a few dings on the top of the helmet as they are very prone to.
Maker marked Mc in a circle. I suspect this is likely to be McCord, a prolific manufacturer of helmets during the war.