The secretive philanthropists who bought letters by Sir Donald Bradman at an auction sustained the mystery today by using a Melbourne boy to return them to the cricket icon's family.

The five letters, written to former Australian captain Greg Chappell during the 1970s and 80s, were handed to Bradman Foundation director Richard Mulvaney during a luncheon at the Melbourne Cricket Ground (MCG) to mark Sir Donald's birthday today.

The anonymous group of businessmen paid $25,850 for the five letters at a Christies sporting memorabilia auction in July, amid outcry that the letters were being sold.

© 2001 AAP