According to Sean Saar from Sunshine Mint, as reported at last summer's convention, the transition from .60 to .50 troy ounces started in 2002 and continued through 2007, depending on when casinos made new dies. The best way to tell on the questionable ones between 2002 to 2007, is to weigh the strike. If the strike weighs 1.2 (or slightly higher) troy ounces, it is almost certain to be a .60 silver center. If the strike weighs in the 1.08 to 1.15 range, it is most likely the .50 center. You have to have an accurate troy ounce scale, but when you weigh them it's fairly easy to tell.

Dave.
SS-1232