I believe this reverse to be from the same sculpture type as the Dionysos Comarmond, which is in the collection of the British Museum today.
That bronze copy was excavated in Chessy, France and dates to the end of the 3rd century BCE. The statue was first in the possession of Ambroise Comarmond, who was an important party in the foundation of the archelogical museum in Lyon.
In that statue, there is no thyrsos and Dionysos holds the Kantharos at his hip. However, the statue’s left arm is in the same position and held slightly open, so it seems likely that a thrysos originally existed. The other hand I believe is engraver’s license, since the sculpture here is shown at an angle and the kantharos would otherwise not be discernible, so it was carved slightly away.
Given the style and the age of the Comarmond copy, it seems logical that the original dates from the 4th century BCE. I am unable to guess who the original sculptor was.