Ahh, I always wondered what it is! Does someone know the reason why they used an anchor?
Ok according to the story that a friend that knows more of the Seleucids than me... the anchor is like also the symbol of Apollo.

The mother of Seleukos had a dream with Apollo and when she woke up she got this ring with the anchor symbol.

When Seleukos was born, he also had this mark on his body, but his mother never told him until he marched with Alexander to the Persian expedition.