It looks like an ordinary detail, and instead it is a clock: that pierced tongue is the gnomon of a sundial. When the sun is at its zenith, a dot of light passes through the hole and descends onto the engraved line, which at the bottom ends in a little bell — or an upside-down lotus flower, depending on the eyes of the beholder. It does not mark all the hours: it marks only solar noon, the exact moment when the sun is highest.