There’s a scene from the movie Sahara which is both cliche’ & apt:
The character tells another, “Coins travel” referencing skepticism about a key movie plot.
That’s absolutely right. Coins prove nothing because they can trade multiple hands from the point of trade origin, and I’m sure that even a millennia ago there were coin collectors.
Intriguing nonetheless.
We know the Silk Road was operating during Roman times. So Roman coins got as far as China.
https://www.britannica.com/money/topic/Silk-Road-trade-route
We know that China had trade relations with Japan.
Therefore it is not surprising that Roman coins diffused to Japan via China and probably India.