1983. 40 years ago.
That was the last time that your Kansas State Wildcats were able to sweep the regular season series with the Kansas Jayhawks. It came as the 4th & 5th wins of a streak of five-straight wins for the Wildcats under Jack Hartman. KU would then go on their first significant run in the series, winning 10-straight, before Lon Kruger broke the streak in 1988.
Of course 1988 is maybe best known for the Showdown being played four times in one season, with the Cats and Hawks splitting the games 2-2.