This wasn't a Columbo reunion, this was a spy reunion. Barbara Bain from Mission Impossible; Robert Vaughn from The Man From Uncle; Robert Culp from I Spy and I don't know who the other man is.
Now I remember; Patrick Macnee from the Avengers. By the way Landau and Bain were divorced in 1993. The Diagnosis Murder episode with her was made in 1997.