Lyric discussion by Fuzzypig 

"dark satanic mills" is the name given to factories during the industrial revolution in Great Britain in the 18th Century.

I think the number is a reference to 6 million jews killed in the Holocaust , the vile Nazi regime was the culmination of the what industrialisation had become by the 20th Century, the concentration camps were factories for human slaughter on a horrendous scale. Mechanization and industrialisation, 200 years of progress and that's all what humankind had achieved, wholesale slaughter of its own kind.

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