As one of the flyers points out...

In the event of bombers all TV and FM stations are to cease operations. AM stations are also to go silent and re-configure their broadcast frequency to either of the two CONELRAD stations (640 or 1240) to provide information in short and periodic broadcasts. The idea was to be on-air briefly and rotate from transmitter facility to transmitter facility in an effort to prevent the incoming bombers from getting a 'fix' on major population centers.

Between 1953 and 1963, Congress mandated that all radios made for the US market were to have the Civil Defence logo (the letters C and D in a circle) marked on the radio dial at 640 and 1240. Overtime, as the Cold War waned, this Emergency Broadcast sytem evolved into the current system of natural disaster advisements and recovery.

Jim