Fifty+ years ago, your standard AM dial had those funny triangle markers at 640 khz (kc back then) and 1240 khz. The idea was, if the Rooskies attacked, normal broadcasting would cease, and civil defense information would be broadcast on those two frequencies. They didn't want the Rooskie bombers tuning their direction finders to 1500 and unloading Tsar Bomba when they got to Washington.