What new card does Sydney now have against Barsad? (From Book 3 Chapters 6-10)

Carton has the card of the "fellow-sheep", Roger Cly, alive and well.

"And indeed, now I think again, I have a strong impression that I have another good card here, not yet enumerated. That friend and fellow-Sheep, who spoke of himself as pasturing in the country prisons; who was he?"