In O. Henry's story "After Twenty Years," why was Bob called 'Silky Bob'?

It should be noted that Jimmy Wells does not know that his old friend is nicknamed 'Silky' Bob. O. Henry does not want Jimmy to know the last name of the man wanted by the Chicago police. Bob's last name is never mentioned in the story. Evidently the Chicago police do not even know Bob's last name themselves but only know him as 'Silky' Bob. If Jimmy had seen Bob's last name included in the wire from Chicago, that would have changed the whole story. Jimmy goes to meet Bob without any idea that Bob is the man wanted in Chicago. There would have been no picture of Bob in the wire because there were no facilities for sending photos or even sketches by wire in 1908. That wire does contain a general description of Bob, plus two specific identifying features. Jimmy recognizes these features when Bob lights his cigar.

The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a little white scar near his right eyebrow. His scarf pin was a large diamond, oddly set.

The wire from Chicago, in addition to describing the little white scar near the right eyebrow, would have contained a detailed description of the diamond scarf pin. O. Henry only states at that point in the story that the pin is "oddly set," but that is to avoid calling too much attention to it and arousing the reader's suspicion that it must have some important significance. The wire would have specified exactly how the scarf pin was set and what made it so different from all other scarf pins. For example, it might state that the diamond was surrounding by little rubies and set on a gold base resembling a four-leaf clover--or whatever. The diamond scarf pin, more than the little white scar, would be an unmistakable indication that the man in the doorway was 'Silky' Bob.

Bob was called 'Silky' Bob by the police because that was the only name they knew him by. Bob was called 'Silky' Bob by O. Henry because he wanted to avoid giving him a last name. If the wire from Chicago had contained Bob's real last name, then Jimmy would know in advance that his old friend was the wanted criminal. How would this have affected O. Henry's story? For one thing, Jimmy couldn't have kept the appointment and listened to Bob at length without revealing his own identity. It was important to O. Henry's plot, not only that Jimmy should not know that Bob was the wanted man, but also that Bob should not know that he was talking to a cop. Jimmy would have known when he read the wire exactly where the wanted man would be at ten o'clock. He would most likely have given that information to his superior, who would have had Bob arrested without Jimmy ever becoming involved. But that was not the way O. Henry wanted to tell his story. The way he tells it is far more dramatic and emotionally effective.