Well, I'm aware, that this isn't quite an "electronics" question, but I'm
hoping to get an answer here anyway.

On an older (guessing some late 60s, early/mid 70s model), light green, big
US Frigidaire refrigerator with icebox in the top area, I'm wondering what
the thermostat settings (in the refrigerator section) mean:

there a three positions labelled Off--A-B-C and if you turn further, three
more, labelled 1-2-3.

So now what are the A-B-C doing and what are the 1-2-3 for?