Actually the 99 cents was a big sales pitch in the late 60s. It was a commercial campaign showing a little boy (much like me) going in and getting the burger fries and shake and handing over a buck and getting the penny back. That sales campaign ran for quite awhile. I never remember it being ‘70 cents,’but I also don’t remember using McD’s much in the early 70s.
It actually reminds me of this X-Files episode in season 11.
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https://www.imdb.com/title/tt6803124/
Seems like an odd promo that would charge prices considerably higher than normal. Oh well, it’s not important.