Coert, if the spike indeed unscrews for a Trichter (which of course makes a lot of sense with a Guard helmet), there is an extra legitimate reason for your helmet having Schuppenketten. If worn with a parade bush (things I heartily dislike, by the way, and in my
real collectors days used to describe as: "das haariges Ungeheuer" for hair of "gerupftes Huhn" for feathers

), the helmet would also be equiped, at least for the occasion, with Schuppenketten.
So depending on possible markings (or lack of same!) there are various options. Markings might indicate a Guard Train helmet in which (as already suggested)
round Schuppenketten would be required. Markings might also indicate a foot Guard regiment, where the present
flat Schuppenketten is alright - at least under certain circumstances. And if there are no markings at all (as the photo's seem to suggest)... well, consider it a private purchase (Eigentumsstück) that was bought with Schuppenketten. There you go

!
For value... I've been too long away from serious collecting to give you an educated opinion. I can only tell you what I myself would be willing to pay if I had the money: I would gladly lay 900 - 1000 Euros on the table and consider myself a lucky bahstud. But again: that is my
personal evaluation of what the helmet would be worth to
me - it may have no sensible relationship to its "general collectors value".
Good looking helmet, mate!