What does Darnay see on a soiled letter handed to Mr. Lorry? (From Book 2 Chapters 21-24)

Darnay claims to know the man and promises to deliver the letter to him. He opens it, and it is a plea for help from Monsieur Gabelle, who has been imprisoned after all. Darnay feels justified in having renounced his title, but he worries that he did not settle affairs in the manner that he should have, and he resolves to go to Paris.