Candidate for a suicide mission to act as a decoy while another agent delivers CONTROL's new code to a base in Greenland. His status as a married man with kids prevents the Chief from giving him the assignment which eventually goes to Maxwell Smart.
[Episode #37: "The Decoy"].
Note: The Chief actually identifies two different agents as Agent 58 but it matters little - both are married; one has three kids, the other four. Both roles are uncredited but one of the actors is recognisable as Norman Burton (seen on the left in the photo).