A long time ago, the threat of fire was a very big deal. You can imagine why with all of the closely dense population. Sometimes to stop a fire from spreading, a terraced living quarter that was not burning would be demolished in the path of a blaze as a sort of “break” hoping that the fire would not jump the expanse.
Well, obviously, people might want to insure their building so that at least they would have some recourse to getting back into a residence. (Insuring the contents was a later idea). Hence the Sun Office. It was an insurance company that placed these kinds of disks (click on the picture to make it bigger, then click on it again, to zoom in)
up high between the windows of the first floor (to you Yanks, that means the second floor).
There are tales of where a volunteer fire company might come out to a residence or building and if unable to find the metal disk, would just let it burn.
I wonder. But I really do not know.