If memory serves, McDonald’s Filet-o-Fish grew out of a desire to provide Catholics with a menu option on Fridays. Prior to that, the restaurant had tried (and failed) to market a sandwich involving a ring of grilled pineapple or something like that.
You've got one sharp memory! Absolutely correct but bad timing for McD. Shortly after they introduced the Filet-o-fish, the church announced that one could eat meat on Fridays ... IF ... they sacrificed something else. Unfortunately, most catholics only heard the first part of that announcement, and celebrated by slapping a steak on the grill :-)
If I recall the article in the Catholic school magazine we used to get, there was a McDonald’s franchisee in a mostly Catholic neighborhood who was losing customers on Friday. He suggested he serve fish, the HQ said ok, and a star is born.
On another note, the same magazine ran a several part story about a man running for president. You never saw his face until the end - when they showed it was a black man. This would have been back in the 1960’s.