Thanks Al. If I correctly understand some info from the Consumer Product Safety Commission pages, it seems that the actual CHOKING HAZARD warnings were not required until after the Child Safety Protection Act of 1994. Although, I do believe that age recommendations (e.g., AGES 3 AND UP) were required earlier.
After 1 January, 1995, marbles required the warning: CHOKING HAZARD - This toy is a marble. Not for children under 3 yrs.