One for the experts, for sure.
Don't be too certain that it's British, however: the Americans wore the same pattern steel helmet back then and liked to paint Divisional signs (which I feel this may be) on them; I've just come across a photo of a US 5th Division soldier's helmet with the Red Diamond painted on.
Black diamond... no luck so far, except that it seems the British 7th Battalion North Staffs had a black diamond shoulder patch, but on yellow, I think.
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