In the days of linotype machines for setting type, multiple pages of copy contained the footer “-more-” at bottom of each page. The end of the copy on the last page carried the footer “-30-“. That told the typesetter in a definitive fashion that he had reached the end of his copy. Why “30”? No one I have ever asked knows. But the designation “30” has stuck as an indication of “the end”.
Dr. Bob adopted this convention years ago when he was a neophyte broadcaster.
Hope this answers your question. Thanks for listening to us and thanks for taking the time to write.
Paul L. Gleiser