The red wedges at 15 and 45 minutes past the hour were for the international silent periods where all stations were to monitor 500 Khz.  The short red marks were to indicate the timing of actual arm signals when they had to be keyed manually.  Four second dash followed by  one second space every five seconds,  for one minute.  An autoalarm would go off if it received four or five correctly spaced signals.

USCG and USN used Chelsea radio room clocks as well. Only circuits on 500 and 2182 kHz would need the clocks with the silence periods marked on them. There was an International Telecommunications Union treaty that required such clocks in radio stations guarding those frequencies.