AR23 - Valerian and Gallienus L Ϛ

Year 6 of the joint reign — rendered as L Ϛ in the regnal dating system Alexandria maintained long after the rest of the empire had abandoned it — places this tetradrachm squarely in the period before catastrophe separated the two emperors permanently. In 260, Valerian was captured by Shapur I at the Battle of Edessa, the first Roman emperor ever taken prisoner by a foreign enemy. He died in Persian captivity, and the joint coinage ended with him.

The Alexandrian mint maintained its own billon tetradrachm standard, effectively isolated from Rome's currency reforms throughout this period.