"CD Marks"?
"Compact Disk"? Nope. 
During the height of the Cold War era, between 1953 and 1963, AM radios made in U.S.A. were required to have two little triangles (or triangles within a circle) on the dial, indicating the U.S. Civil Defence (or CONELRAD - CONtrol of ELectronic RADiation) frequencies, at 640 kHz and 1240 kHz.
CD Marks

In the event of a national (U.S.A.) emergency, all other broadcasting would be suspended, and only these frequencies would remain on air, at low power. The transmissions would originate from locations that changed, so as to prevent any attacking enemy from homing in on the signal. Although it was instituted in 1951, it was not required on new radios until1953.

In 1963, the CONELRAD system was replaced by the Emergency Broadcast System (EBS), and radios were not required to have the CD marks.

For more on CONELRAD, see Cold War Civil Defense - CONELRAD.

For more about the the Cold War and the media, see the CONELRAD: All Things Atomic.

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