The hook on this doorbell marks it as a unicorn. Many building entryways evidence telltale technological change, such as the addition of a gate, the replacement of a knob, the widening of a passage. In this case the doorbell retains the vestigial communication appendage on which once hung a mouthpiece. Several generations have passed since it would have been in use. Minus that hook (which I tried to photograph at an angle), the circle above it would be mistaken for a two-way speaker/microphone, whereas it was only ever used for the former.
An ongoing series cross-posted from instagram.com/dsqt.