Who visits Armpit while he is doing some plumbing and what do they want?

While working on a complicated piece of underground plumbing, Armpit is visited by Felix, Moses and the beaten-up X-Ray. Felix tells Armpit that he's been visited by Det. Newburg who, thanks to X-Ray, thinks he's involved in the selling of the forged tickets. He tells Armpit he he wants to buy Kaira's letter from him, having learned about it from X-Ray, and says that if he gets the letter, he keeps quiet about what he knows about X-Ray and Armpit's involvement with the tickets. If he doesn't, he talks.