Well, mystery solved...no arrowhead (amazing how light can trick your eyes) the marking is in fact 'HH / 0 355', HH being Harrison and sons & Howser Ltd. They started manufacturing helmet shells in 1915.
Well, mystery solved...no arrowhead (amazing how light can trick your eyes) the marking is in fact 'HH / 0 355', HH being Harrison and sons & Howser Ltd. They started manufacturing helmet shells in 1915.