From PCGS Library

"To date only five 1968-S Proof half dollars
 with inverted Mint marks have been found"
     by Ken Potter - March 12, 1999

The Top 50 U.S. Coins of All Time
     by Ed Reiter - December 27, 1999

A Clad-Edged Investment
     by Ed Reiter - November 19, 1998