Very interesting! Maybe that book has some clues.

I think Mary's husband may have been another slave named Adam who appears in that inventory and later inventories, but we cannot be sure. There was another Adam Adams born about 1760, a "free person of color," who served in the Revolution and lived in adjoining Charles County. (I've updated my site with all the latest info I have).

On thinking about this family further, I realized that the two-year-old "Mulatto" boy in the inventory indicates that Mary was married for at least two years before 1682. So her marriage should have come under the same law as applied to Eleanor Butler in adjoining Charles County; ie., she and her children should have been slaves for life. (The law was repealed in late 1681). Perhaps the St. Mary's County court--unlike the Charles County court--allowed the family their freedom based on the new law.
Paul