I believe what August showed Neil in his box is the drawing of the dagger. We saw the drawing when Rumple searched August’s room. We didn’t see what Neil saw in the box because it would have given away that Neil is Bae. Bae would be very familiar with this dagger since he and his father – Rumple – talked about it at length while looking at it in “The Return”. Seeing this drawing completely convinced Neil/Bae that he had to listen to August. Somehow this means August knew this would be the only way that Neil/Bae would believe. August probably got this drawing and the knowledge how to find Bae from the Blue Fairy, who I believe also created the fairy tail book for Henry.
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Don't grow up. It's a trap.