A Harper Lee Mystery

Here's a scan of the front cover of an early copy of Harper Lee's To Kill a Mockingbird (Philadelphia & New York: J. B. Lippincott Company, 1960. Inside flap -- Third Printing; inside verso of title page -- Third Impression).

Provenance: my mother picked this up somewhere along the way; I think it was in North Carolina at a used bookstore or garage sale, probably in the 1970s. 

The inscription on the front endpaper, in blue ink, reads:

"To Mack and Zada Miller 
 with my best wishes, 
 Harper Lee

                           September 2, 1960"

To Kill a Mockingbird was first published on July 11, 1960. So the mystery element is: who were Mack and Zada Miller and what became of them?  And: besides autographing this copy of her beloved novel, what was Harper Lee up to on September 2, 1960? And where was she?    

Today's Rune: Defense.