I bought this one from Joe last year after SOS and remembered I never posted photos of it. We think it's from around 1868 or so, perhaps private purchase (no unit marks anywhere). Take a look, see what you think.
Francis was good enough to remind me that the rounded non-stepped visor is post 1889. That would probably place the service life of this between 1889 and 1897, unless it's just missing a cockade. Joe probably told me this, but I changed it around in my brain with my tendency to transpose numbers (the reason I'm an artist, rather than an architect!). Maybe a re-worked helmet?