What does Carton find in Dr. Manette’s coat? (From Book 3 Chapters 11-15)

From the text:

Carton stooped to pick up the coat, which lay almost entangling his feet. As he did so, a small case in which the Doctor was accustomed to carry the lists of his day's duties, fell lightly on the floor. Carton took it up, and there was a folded paper in it. "We should look at this!" he said. Mr. Lorry nodded his consent. He opened it, and exclaimed, "Thank God!"