The Iron Age coins—known as potins due to the copper, tin and lead alloy used to make them—each measure about 1.2 inches in diameter. They show stylized images representing the Greek god Apollo on one side and a charging bull on the other. In England, potins have mostly been found around Kent, Essex and Hertfordshire. People in Britain may have begun making the coins around 150 B.C. The earliest versions were bulky disks known as Kentish Primary, or Thurrock, types. Comparatively, the newly discovered potins—now dubbed the Hillingdon Hoard—are of the “flat linear” type, which uses simplified and abstracted images....