This particular McCallum callback happened in the season 2 episode "The Meat Puzzle." The title refers to a jumble of body parts that Ducky and his assistant, Jimmy Palmer (Brian Dietzen), have been working to reconstruct and identify. They make a breakthrough in the project when Ducky identifies a distinct body part as belonging to Michael Grant, a district attorney whose cases Ducky had worked on multiple times in the past. When the other bodies turn out to be all related to a particular case, one in which Ducky's testimony helped send an accused murderer to prison, the team becomes worried that Ducky could soon become a piece in the grim puzzle.
This leads to an ongoing stakeout of Ducky's house, which gives viewers some fantastically funny scenes with Ducky's eccentric mother, Victoria Mallard (Nina Foch), as well as some personal insight into the past of the beloved character. At one point, Agent Kate Todd (Sasha Alexander) asks Gibbs, "What did Ducky look like when he was younger?"
Gibbs smiles glibly before responding, "Illya Kuryakin." Although the name might be totally lost on viewers who aren't familiar with McCallum's resume, it is a pretty smart meta-joke for the series to include.
Illya Kuryakin is the name of a character that McCallum played on the '60s espionage series The Man From U.N.C.L.E. And even in the NCIS universe, Gibbs isn't pulling some obscure reference to make the comparison. Illya Kuryakin was an immensely popular TV character of the era and playing him rocketed McCallum to stardom back in the day.
