When does Jimmy recognize his friend Bob in "After Twenty Years"?

Jimmy recognizes his old friend Bob while he talks to him in the doorway of what once was Brady's restaurant. As Bob lights his cigar, Jimmy recognizes the "pale, square-jawed face with keen eyes and little white scar near his right eyebrow."

This recognition is not only of his old friend, but also of a wanted man known as "Silky Bob," a face Jimmy explains in his letter to Bob "of the man wanted in Chicago." And, because his friend is now a criminal and Jimmy is a policeman, they can no longer continue their old relationship. Still, Jimmy does not have the heart to embarrass his old friend by arresting him then. So, he pretends to not recognize Bob.

Later, however, a man posing as Jim comes across the street to Bob, and they walk together, talking until the man passes under a street light. Having wondered how Jimmy had grown taller, Bob's wonderment turns to astonishment as he notices that Jimmy no longer has a Roman nose. This man's is a pug. "You're not Jimmy Wells," he abruptly says. The other tells Silky Bob he has been under arrest for twenty minutes and shows him a note from Jimmy Wells that reveals his recognition of Bob:

....When you struck the match to light your cigar, I saw that it was the face of the man wanted in Chicago. Somehow I couldn't do it myself, so I went around and got a plainclothesman to do the job.

As Bob has said, Jimmy is "the truest, staunchest old chap in the world" as he saves Bob some embarrassment.