The button at 9:00 is a corrector for the 9:00 sub dial. It is not a 24 hr dial; rather it is a tide calendar. The quadrant of the dial with the crescent is positioned according to the phase of the moon, via the corrector. Tides times are shown by the blue segment of the sub dial relative to the marked chapter surrounding the disk. For example, in the image attached to the original message, tides are indicated at roughly 12:15 - 6:15.