If you grew up in the United States during the Cold War era, you are probably old enough to remember the US Civil Defense (CD) frequencies marked on AM (medium wave) radios. A "CD symbol" with simple white or red triangle highlighted the frequencies 640 kHz and 1240 kHz. These designated frequencies aided listeners in tuning to CONELRAD stations.
I recall from the 1950s and 1960s -- especially the early 1960s -- radio stations (TV stations too) would occasionally interrupt regular programming and test the Emergency Broadcasting System (EBS). It would begin with the message, "This is a test. For the next sixty seconds, this station will conduct a test of the Emergency Broadcast System. This is only a test." It was followed by a shrill sound that combined sine waves of 853 and 960 Hz, an interval signal so unpleasant it attracted the collective attention of the public. Decoders at relay stations would sound an alarm, alerting the station operator to the incoming message. After the test an announcer would state, "If this had been an actual emergency, you would have been instructed to tune to one of the broadcast stations in your area."
The purpose of the test was to allow the US Federal Communications Commission and broadcasters to verify that EBS tone transmitters and decoders were functioning properly. In addition to the weekly test, test activations of the entire system were conducted periodically for many years. These tests showed that about 80% of broadcast outlets nationwide would carry emergency programming within a period of five minutes when the system was activated. Over the years the message and procedure changed, but essentially the content remained.
Today, you will find no radios with this symbol. It is the relic to a dreadful past, hopefully gone forever.
Source: South East Asia DXing