Originally I thought this was a strange wisecrack about the sexual habits of the Scottish people, but it's really about people who come into the theatre late. The first paragraph of the advertisement contains a string of references that I will explain below.
Just when the body is discovered or the third one from the left looks your way, the Scotts arrive. Their seats are never Robinson Crusoes (two in the aisle, if the old lady from Dubuque should inquire.) Men, otherwise kindly, grow apoplectic and stout ladies make "tut-tut" sounds. But no one knows the reasons for the tardiness of the Scotts. We do.In true theatrical form I'll leave the explanation for this baffling mystery until the end.
* "Just when the body is discovered..." Obviously a reference to the inevitable climax of the first act of a mystery play: after the scene has been set and the characters introduced, one of the characters is found dead. Mystery plays were extremely popular in 1927, though in 1928 they seemed to be going on the wane.
* "...or the third one from the left looks your way..." A reference to those who tried to catch the eyes of chorus girls, a line of whom appeared in every musical show. Stories and articles from the '20s often make references to one particular girl who "stood out" from the line as prettier, more intelligent, more talented, or more tragic than the rest. She's the one whose eye you tried to catch...just before those darned Second Act Scotts barged in.
* "Their seats are never Robinson Crusoes..." I've never heard this one, but it's explained in the text. I imagine the aisle seats were called "Robinson Crusoes" because they were sort of distant and stranded from the rest, but I can think of lots of other equally bad explanations if you care to hear them.
* "...the old lady of Dubuque..." This was a running gag since the inception of The New Yorker. Their editorial policy was that the paper "wasn't written for the old lady of Dubuque," implying that their articles would be sophisticated. The OLOD got referenced a lot in the paper, particularly by advertisers trying to be clever.
SO WHY WERE THE SCOTTS LATE ALL THE TIME, ANYWAY? Because they were too busy eating really good home-cooked meals made of Guasti Sherry! This was bona fide alcohol, but so salted that only a homeless anchovy would dare drink it.
As a side note it's unusual to read an advertisement that basically says "these people are REALLY ANNOYING...because they use our product."