Even Bob's first name is not mentioned until near the end of the story, and the nickname "Silky" Bob is mentioned only once. This is when the plainclothes detective tells him,
You've been under arrest for ten minutes, ‘Silky’ Bob.
Bob's last name is never mentioned in the story. O. Henry has some reason for not wanting to identify Bob by his name. This is undoubtedly because Bob is continually on the lam and probably uses many different names. Jimmy Wells, of course, would know Bob's last name, but he cannot call him by his first name, last name, or nickname when he is talking to him in front of the hardware store because he has quickly decided not to reveal who he is or that he knows who Bob is. Jimmy makes this decision when Bob lights his cigar and Jimmy sees that his old friend is the man who is wanted by the Chicago police. From that point on, there is no exchange of names. Bob speaks of Jimmy Wells, but he does not know he is talking to Jimmy Wells. Bob is called "the man" and "the other," and at one point he is called "the man from the West," but he is not called Bob until the arresting officer arrives on the scene and calls out:
“Is that you, Bob?” he asked, doubtfully.
The plainclothes detective only knows the name because Jimmy Wells has told it to him. O. Henry could not have had Jimmy addressing Bob by name because Jimmy was pretending he didn't know him. Bob would have had no reason to introduce himself by name to a strange cop unless asked to show some form of identification. Both Jimmy Wells and Bob's identities are unknown to the reader until near the end of the story, which is O. Henry's intention because he wants a surprise ending.
Bob probably got the nickname 'Silky' Bob for two reasons. One is because he is apparently a smooth talker, as he demonstrates when he encounters the cop while he is standing in the hardware-store doorway. Bob is slick. He is like silk. Another reason is that Bob has a taste for luxuries, as demonstrated by the diamond scarf-pin he is wearing and his diamond- studded pocket watch. He probably also favors silk clothing. This would easily get him the nickname 'Silky' Bob in underworld circles. Crooks generally go only by their first names or a nickname to avoid being caught.
The word 'Silky' appears only once. The plainclothes detective has to show that he knows the man he is arresting. He has to make a positive I.D. This is why O. Henry has him say,
You've been under arrest for ten minutes, "Silky" Bob.
It may be that the Chicago police do not even know Bob's last name but have identified Bob in their telegram as "Silky" Bob. This would be sufficient for the New York detective to arrest Bob, although Jimmy Wells could supply Bob's last name when booking him. Bob might have many reasons for wanting his last name to be unknown, and Jimmy could cause him a lot of trouble all over the Midwest when he reveals what it is. If Bob is wanted by the Chicago police, he may very well be wanted by the police in a lot of other towns.