Sometimes leaving us in the dark leads to a more entertaining ending. It worked in "Columbo Goes to College" years later too.
As for "Murder Under Glass," it was not too bad an episode. Paul Gerard was really full of himself, and the writers made him get with he deserves. First, his cohort steps out on him when "Irene DiMilo" says she is quitting. Then, Columbo gives it to him good in the final scene. A terrific Columbo line: "I respect your talent. I don't like anything else about you. The dressing is perfect."
Cassa - good points. I think that it is the execution of the murder that is the flaw at the heart of this episode. There are no other plausible suspects in the episodes and no attempt to frame anybody or suggest some random unseen person. This forces Columbo to almost by default to focus on Gerard and concentrate on how the poison got inside the wine bottle.
David - that final scene is good. I particularly like the very last line and the ambiguity of Gerard's "I wish you'd been a chef."
It just occurred to me that Columbo and Horace Rumpole share similar pedestrian eating habits (bangers & Chateau Thames Embankment versus chili & iced tea), and upscale cuisine has been central to at least one of their mysteries (I believe it was "Rumpole a la Carte"). I'm trying to remember if other detectives were associated with the ordinariness of their food preferences . . . many such as Nero Wolfe and Hercule Poirot had gourmet tastes, but how many established rapport with their fans by their "workingman" eating habits?