This symbol is also known as the Seal of Saturn. In 3.04 Sin City it is revealed, that the name of the Yellow-Eyed Demon was Azazel. John's use of the Sigil of Azazel to call the Yellow-Eyed Demon in 2.01 In My Time of Dying suggesting that John knew all along what the demon's name was.
This is the sigil of Azazel (Zazel): Is that the one you were expecting? No? Maybe you're used to seeing this: No worries. It's a common mistake.