Further reading indicates that the zinc sulfides may have been used as a base to which other things were added. It may be possible that radium was used in the manufacture of the card.
Perhaps it might be good to contact your states Radiation Control program
http://www.health.ri.gov/programs/radiologicalhealth/index.php (for Rhode Island)
or your regional EPA radiation office.
Region 1 Boston, Massachusetts
Anthony Honnellio
Office of Administration and Resources Management
Ph: 617-918-1456
They would be able to tell you how to have it tested. If it does have radium or other radioactive materials, they would also be able to tell you what the safety risks are and describe the appropriate handling and storage techniques.
