how do you know how many people took the test? They don't need to use the super-secret fallback test to keep every black person from voting, the test was if all the other methods to keep them from voting didn't work, and then you didn't necessarily use all the test, you used some of the test, just enough to say they failed, and then what? Do you register the test somewhere?
Since the literacy test was used at the discretion of the authorities in charge of the vote they could choose who to give it to based on how likely they were to get away with using it.
I mean if you know the black people in your district will vote for a particular party you probably don't actually want to keep the black people in your district from voting, you want that party not to win because otherwise the party might help the black people living in your district.
If there are 1800 black people and 1100 white people that can vote in your district, then maybe you only need to keep 900 people from voting to be safe.
So then you announce you will be checking outstanding warrants at the polls, 600 people don't show up. You only need to keep 300 people from voting! So you start giving literacy tests to black voters but letting the white voters through - how many black people you think you will actually need to give that literacy test to before the rest of them wise up that you aren't going to be letting them vote?
I'd say maybe 20.
Now how many of them going to get copies of that test to do something about? What if you don't want to give them a copy of the test? How they going to get that copy of the test?
I'm sorry but I think this kind of thing would be pretty under-documented, just like most crime. I'm agreeing you can't keep it thoroughly hidden but hidden enough that it is difficult to say with any specificity this was the actual test used in that district on that day to fail these people.
on edit: removed something that was probably a bit rude, sorry, was going through some problems with kid at the moment and frustration transferred to my writing.