(Penises are crucially involved in the first of these ads, though they aren’t actually mentioned, much less depicted. Still…)
It’s crude, about men’s underwear, and you probably don’t want to go there, but there it is, or at least was, at least in someone’s commercial imagination. This ad from ca. 1969 (thanks to Peter Korn):
(#1) The model is alarmingly wasp-waisted; that can’t be healthy.
So: not a fly, but a stretchy hole to push your penis through, for the exigencies of the moment. An innovation that seems not to have caught on, or even found its way into stores. A lost inspiration of the 1960s.
(The Regency Square company appears to have vanished long ago, at least under that name. The building was a nondescript office building, not a shop or showroom.)
From the same source:
(#2) Sp. cortitos ‘shorts’
And a whole assortment of slightly dubious menswear:
(#3) Probably worth it just for the lounging cowboy figure; though stretch denim really doesn’t work that way, it’s just stretchy (from elastene) denim
These ads are designed to appeal to male vanity. Their target audience was probably gay men, who are comfortable being objects of sexual desire. Though I wonder how many of these garments they sold, and to whom.