In my career as a librarian I had to deal with many patrons like Bert. That's a difficult situation.
You can't produce 'proof' that doesn't exist. At the same time, you just can't brush someone off and tell them they're nuts, although you would love to do so.
Perhaps Bert has heard about the 'Harlem Renaissance' and is using that as the grain of sand on which he builds his pearl of a 'Golden Age'. Perhaps he added together one plus one and came up with umpteen-gazzilion. That's usually the way these things work.
These folks are perfectly sincere in their beliefs but they're set on the idea that the PROOF must be out there. If you tell them that it there's no evidence for their beliefs, they will tell you that, 'Absence of evidence is not evidence of absence'.
The fact remains that there IS absence of evidence and at the best librarian in the world can't cobble something up out of thin air to satisfy the pipe-dreams of someone like Bert.
I feel for Bert. I want what he wants to be true. The problem is that, although many people have tried, history can't be altered. History will win out in the end.