I think this steel helmet was made by Rubery Owen and Co, an engineering firm based in Darlaston (W Midlands) and Leeds, that made these helmets between 1939 and 1943. "RO.Co." is stamped into the metal on the underside next to the date stamp.
This helmet is dated 1939. There appears to be a "1" or an "I" stamped above the year.
This type of steel helmet was issued to the British Army, Home Front and National Defence at the outbreak of WW2.
I think the helmet should be black all over but it has developed a camouflage of its own, with uniformly spaced rust spots all over the top.
The metal is non magnetic
It has an elasticated (sprung) webbing chin strap. The springs are still stretchy. The webbing is coming detached on one side and the strap appearss to have been mended at some time, judging by the coloured thread and frayed edges. Please look at the photos.
The liner is secured with a large brass screw and nut. The liner may be a MK1 style liner? The liner is threaded through with a brown shoe laces to allow for some adjustment. The "rivets" holding the liner together have gone green.
Diameter front to back 31 cm; side to side 29 cm; weight 1.1 kilo (2.8 lbs). All measurements approximate.
From a smoke free home. Any questions please ask. Tracked delivery.