What was the device called which Faber had given Montag in order to communicate with him?

In Part Two "The Sieve and the Sand" of the novel Fahrenheit 451, Montag travels to Faber's house trying to find meaning in the texts he is reading. Faber is hesitant to help Montag because he fears that the government will arrest him. Montag suggests that they make copies of books and distribute them. Then, Faber makes the suggestion that they should "plant" the books in the homes of firemen to destroy the entire system. Faber tells Montag he is only joking, and then admits to him that he is a complete coward. Before Montag leaves, he asks Faber if there is any way he can help him with Captain Beatty. Faber opens his bedroom door and leads Montag down a small hallway to a room with tools, gadgets, and various mechanical devices. Faber gives Montag his two-way listening device called the "green bullet." The green bullet resembles a Seashell radio and allows Faber to not only communicate with Montag but also listen to his conversations. Later that night, Faber reads Montag the book of Job via the green bullet.