According to Matsson (The Gods, Goddesses and Heroes on the Ancient Coins
of the Bible Lands) the world-encircling river Oceanus was used by Helios
to glide back to his
starting point in the East during the night so he can set out again to ride across the sky in his
Dictionary of the Classical World notes that the sun and stars were believed to set in and rise from Oceanus.
However, I can't find any reference to Oceanus being used this way by the moon.
Another option is that it is Oceanus' wife Tethys. Though I can find no record of her portrayed in this form, there is a myth involving her and Callisto who is turned into the constellation Ursa Major. Could the stars represent Ursa Major??