A decades old case of threatening letters sent to a Circleville woman's address will be featured tonight on the CBS-TV show "48 Hours". The letters, reported to have numbered over a 1,000, were mostly sent from an unmarked Columbus address. The contents of the letters alleged a Circleville school bus driver was having an affair with the district superintendent. The writer also warned of possible harm coming to the driver's daughter if the alleged affair did not end. The letters started arriving at the bus driver's residence in the middle 1970's, and no one has ever been charged with writing them. The show airs at 10:00 tonight on CBS Television.