Possible anachronism

When Thale goes to get herself a can of mystery meat for food, she opens the bottom of the can. And when you see the other end, you can see that it has a ring for opening it without a can opener. These cans had expiry dates from the 80's. I am quite sure (though not 100%) that in the 80's there were no cans (at least not in Norway) with that kind of opening.