Thanks for that.

This report is from the very same locality as the pit Robeson visited:

"One of these villages, whose foundations can no longer be traced, occurred in the immediate vicinity of Niddry Mill. It was a wretched assemblage of dingy, low-roofed, tile-covered hovels, each of which perfectly resembled all the others, and was inhabited by a rude and ignorant race of men, that still bore about them the soil and stain of recent slavery. Curious as the fact may seem, all the older men of that village, though situated little more than four miles from Edinburgh, had been born slaves. Nay, eighteen years later (in 1842), when Parliament issued a Commission to inquire into the nature and results of female labour in the coal-pits of Scotland, there was a collier still living that had never been twenty miles from the Scottish capital, who could state to the Commissioners that both his father and grandfather had been slaves, that he himself had been born a slave, and that he had wrought for years in a pit in the neighbourhood of Musselburgh ere the colliers got their freedom... "

https://randomscottishhistory.com/2019/05/09/iv-serfs-colliers-and-salters/