My recollection is that the "key" there allowed a teacher to give out multiple tests with questions in different orders, and then you would mark the letter for the test that you were given in that section. So test A would have questions in order 1 2 3 4 5, while B would be 2 5 4 3 1 and C would be 3 4 1 5 2. They could also change the letter of the answer, or really whatever you imagine.

Presumably it reduced people copying answers from their neighbors.