Cheddar was never known as “American cheese” – although, I assume that all or most of the cheddar we ate was of domestic origin. Now-a-days we make cheddar, some of which is every bit as good as the best English cheddar.
The main difference I’ve noticed–in earlier decades or now–between “American cheese” and domestic supermarket “cheddar” (which is to cheddar as Gallo “Burgundy” is to the French product although there are, indeed, artisanal cheddars produced in the US that are quite good) is that the “cheddar” is “sharper” and the American has a softer, more rubbery texture. But this may be mostly a matter of aging. And the mildness of flavor and softness of texture is not necessarily a bad thing if you are looking for a cheese that will melt well and not overwhelm other flavors.
My point, though, is that the dividing line between what is called “cheddar”and what is called “American” varies by manufacturer. And in the 50’s and 60’s I really don’t recall much, if any, “cheddar” in supermarkets. What they had besides “American”–and I know this because my Dad loved it–was “sharp cheese” (so labeled). That was often made in Wisconsin but I don’t know if the labeling was a regional thing (Ilived in the DC suburbs).