Beautiful picture Kushanshah!
In the case of these classical silver coins from Istros, a modern interpretation of the reverse which I am aware of, might sound perhaps less controversial than the multiple interpretations I read for the twin-face obverse.
The coin reverse depicts an eagle above a dolphin.
On many references, it is claimed that the eagle is actually attacking the dolphin - gripping it with its talons - a view which I donīt support nor reject.
Well, a giant eagle gripping and lifting a dolphin in the air would be quite an astonishing sight to behold in real life
but this comes along with an interpretation:
Dolphins were common in the Black sea (And although endangered by man, they still are today).
Since they were a frequently seen symbol in the Black sea region, they are believed to have had some special meaning to the area.
The eagle, on the other hand, was closely associated to Zeus (Aetos Dios), being both his attribute and personification, and a frequently seen symbol on greek coinage.
This scene, thus, is thought to be a representation of the greek control over the Black sea region, possibly symbolizing a decisive military victory.
Whether this is truth or pure speculation, goes beyond my limited knowledge.
What it is certain indeed, is that this same depiction has also been used on other Black sea coins, such as the beautiful coinage of Sinope in Paphlagonia:
or Olbia in Scythia: