In the left margin, about 2/3 of the way down... between a couple of alignment marks. But that was 35 years ago... I could be wrong. I always felt like I held an immense power with that knowledge, but never used it!!!
The scantron forms that were in use while I was in high school had a sort of meta-data section, where students would put their name, the date, etc. One of those boxes was labeled 'key'. I always assumed it performed that function, but I never tried it to see.
Maybe the intention was to make it easier for teachers to identify a student maliciousmy marking it?
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.
Maybe the intention was to make it easier for teachers to identify a student maliciousmy marking it?