Said to be the oldest of their kind in the world, dating back some six millennia, they were worked by the ancient Egyptians and later (perhaps) by King Solomon's slaves.
They might have been worked by Egyptian slaves.  
mnli hypothesis: They might have been worked by Egyptian slaves.   premise: Said to be the oldest of their kind in the world, dating back some six millennia, they were worked by the ancient Egyptians and later (perhaps) by King Solomon's slaves.