German Franco-Prussian War Veterans 1870 Iron Cross Door Plaque, Mail Holder?

Originalperiod manufacture. Now this is something I have never seen before. A bronze colored alloy plaque with mounting screws at top and bottom. Affixed to the front is a metal projection which gives about a 1 inch gap from the back of the hook to the face of the mounting plaque. The circular disc on the 'hook' has a gilded edge with a green edged 1870 Iron Cross at center. I have no idea as to its true intention, perhaps to hold the mail or newspaper? Anyway, it's a neat thing...