First of all thank you for the prompt and helpful replies
And after a little bit resarch I agree with most of you: it IS a Fire Police helmet! Reason: the national decal is angled -->German-Helmets.com
I think the helmet is an Aurstiran M16, used after 1938 for Fire Police --> German-Helmets.com (note the position of the splits)
The Helmet looks similar as early SS, but the swastika is on the other side (s. Photo below).