Hi Brian,
My guess would match yours, that it is a device for a cartouche box.
I'm surprised they would place only one screw post on it, instead of either two vertical screw posts, or two horizontal screw posts in order to secure it more firmly to a lid of a cartouche box, and prevent it from turning/pivoting.
Maybe it was for the Baden L G R 109 as Steve mentions, or possibly for the Hessen 115th, or one of their 2 Dragoon Regiments. I'll have to look in my Verlag books to see if I can find a cartouche box photo with this crown device on it.