You might think this is simply an eagle fighting a snake (that seems to be on the loosing end). However, the eagle seems to represent the victory of the Eastern Roman Empire, the snake those laying siege to Constantinople. The catalogue mentions the motif was employed by Alexander the Great in the bonfire for the god (the Wikipedia mentions: the hero. not the god) Hephaistios in Babylon in 324 BC. And it appears in the legion emblem of the Roman army.