It looks like coins have been found on the sea routes that avoided the Parthian/Sassanian empires, but not on the overland routes. I’m guessing merchants exchanged their coins on the Roman/Persian frontier, east of which the Iranian coinage was the standard anyway; but in politically fractured places like southern India (south of the Kushans and Guptas), Roman coinage became the de facto currency of international trade.
So in other words, the distribution of coins outside the empire could reflect a regional demand for mutually-accepted coinage, rather than Roman trade per se.
It looks like coins have been found on the sea routes that avoided the Parthian/Sassanian empires, but not on the overland routes. I’m guessing merchants exchanged their coins on the Roman/Persian frontier, east of which the Iranian coinage was the standard anyway; but in politically fractured places like southern India (south of the Kushans and Guptas), Roman coinage became the de facto currency of international trade.
So in other words, the distribution of coins outside the empire could reflect a regional demand for mutually-accepted coinage, rather than Roman trade per se.