Excellent piece James. I have never seen one in the "flesh" before. Looks to me like your spike top twists off...so from a trichtered regiment. Does this provide any clue as to regimental identity??
So did pioneers, unfortunately for us. I have exactly the same one, same colours inside, unscrewable spike, but totally anonymous, too. We will never know their original affectation...
Great helmet James, congratulations.