Many thanks to all for your comments. These officer early helmets were not Ni-plated, like later models, which makes restoration considerably easier. Only bare steel to be polished (perhaps some of these were actually plated; happy to learn if some of you know...).
On this one even the liner string is original. The leather lining inside the visor has turned light brown, although a tiny amount of remaining dye confirms it used to be green in colour.