Well, he either learned about it during the council meetings as a kid, directly from the BF when she agreed to send him through the wardrobe, or from Neal at some point. After all, it’s suggested that they talked a fair bit more than we initially thought, and that August knew from their first meeting who Neal was. (Stuff we didn’t know until fairly recently.)
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Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing