Who is the other fellow-Sheep at the prison? Why does Barsad say that Cly has been dead for several years? (From Book 3 Chapters 6-10)

Carton threatens to denounce Barsad, adding that he recognizes the man with whom Barsad was talking as Roger Cly. Barsad tries to claim that Cly is dead and had a funeral back in London, but Jerry Cruncher interjects, saying that he looked in that coffin, and there was no body in it. Despite the fact that he grows defensive when asked why he knows this, Cruncher sticks by his assertion, and Barsad gives up and agrees to help Carton.