The cover likely dates from the early twentieth century. Before telephones were common in every home, European cities (including Cologne) established a network of “alarm boxes” in the streets. When a fire broke out, a citizen would run to the nearest box, break the glass, and pull a lever. This action sent a telegraphic signal (in Morse code or a simple digital code) directly to the central fire station, which could immediately identify the location of the box.
The cover protects the communication manhole through which the dedicated telegraph cables of this system passed.
Translation of the text on the cover: Odenthal Brothers Cologne-Ehrenfeld Fire Telegraph