I'd seen them before and at some point growing up around boats I'd heard there were radio silence periods, maybe in a marine antiques store or something. At least with a starting point, I knew where to look.
The two red bands are periods of radio silence during which only distress signals can be transmitted. The two black bands correspond with the green bands on some other clocks, and those two periods were also radio silence to listen for coastal distress signals.
http://www.chelseaclock.com/site_ass...pdf?1315842133
Go down and look at the radio room clock.