American Journalism Review: What's the -30- mean at the end of journalists' stories?

American Journalism Review: "Each October for the past eight years, students in Louise Reynolds' Introduction to Journalism class at Walt Whitman High School in Bethesda, Maryland, have been offered extra credit if they can solve one of journalism's lingering mysteries: Why did reporters for years end their stories by writing '-30-'? 'Journalism is so full of funny phrases and traditions,' Reynolds says. 'I wanted the kids to know there was a long tradition behind each of these terms and style rules and know that it didn't come out of nowhere.'"