Possible Clues:

• Transmission from broadcaster -- in panic all's ignored
• Combination of identifying letters and numbers for a ship, aircraft or transmitter
• Station identification letters
• Ring to stop identifying the Jumbo
• Identifies jumbo for everyone after college and two-thirds of 8 down
• Identifying code of sender or receiver of broadcast message
• A wireless operator's identification
• Unique designation in radio communications
• Mark Bellow from the start is employed by radio station
• Mark Bellow from the start produces ID for radio station