Welcome to the forum! Nice little helmet! One thing you quickly find out about this field collecting is that for some unknown reason Hessian helmets like yours are highly sought after. It is more than a rarity thing just for some reason Hessian is hot
I took the liberty of copying your picture of the Mark and enhancing it with Photoshop. if you object that will remove it. It is tough to read but looks very much like Dragoon Regiment 24 which is consistent with both Hesse and the example we have in our list from the Quinlan collection.
You have started this journey in a wonderful niche! I for one love the research.