Supposedly, it's that very reason they exist in the first place. A Catholic guy owned a McD's franchise in a very Catholic neighborhood and invented the sandwich so he'd have something to sell during Lent.
At least that's what I heard.
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Also it might be a regional difference because I usually find my Filet-o-Fishes to match the image as much as any other burger.
It is. Ray Kroc, the founder of McDonald's, came up w/ a Pineapple burger, which I think was just a pineapple slice on a bun. The guy you mentioned came up w/ a fish sandwich, so Kroc had them compete to see which would be the most popular. I think the consumers made the right choice.