Thanks to Paul for the determination of this currency.
Size: 2.5 cm
Dear Joseph, it is certainly not a Celtic coin nor Byzantine. I think it is a silver drachma of Hephtaliten
(also known as White Huns), and hammered in Central Asia (Afghanistan / Pakistan), around 600 AD.
Such coins are found in the book of M. Mitchiner, Oriental Coins and their values. The Ancient & Classical
World 600 BC-AD 650, numbers 1485-1489.
With the best regards,
Paul