My "source" right now is the museum under discussion. As a major historic center, it has hosted tourists from all round the globe and I am sure among those have included people who had the power to object that claim if they had enough counter evidence. While posting this, I had to confirm the info by looking at the footage on my camera I had of that poster on the wall.

I must admit I was fascinated by the precision. It tallied the numbers from all the major slave ports from the Guinea to Ghana to Nigeria. Considering that this was a very elitist transaction at the time, it is really not surprising that they could have such information. I mean every cargo had to be declared with its contents.