I think you've hit on it right enough! I removed the spike and examined it carefully with a high-power glass. It isn't exactly the same tone, only slightly but it isn't, to all the other metal parts, having a hardly detectable brownish fleck on it that isn't to be found anywhere else. It can't easily be detected just looking at it with the naked eye. All other metal parts match in tone exactly. And with the spike off I can see that the mount is badly deformed on one side, as if struck by a heavy object, the left side crushed downwards, indicating I should think that the helmet was worn without anything on it. In a way that's fine, since I have a 1915 Pickelhaube missing it's spike-now I have one!
I hadn't actually though of the Foot Artillary, as I had only seen it given previously as 'FA', not 'FsA'. So, if that is a Foot Artillary stamp I assume it refers to Thuringisches Fussartillerie-Regt. Nr.18.
Thanks for your assistance. My knowledge of WWII helmets is quite extinsive (I have 60 of them), but not with regards to WWI.
All the best,
Denton