The title can be explained with this sentence from her Wikipedia article, "On 8 January 1634, at the age of 12, she married the 15-year-old Charles, Lord Herbert, eldest son of the 4th Earl of Pembroke and 1st Earl of Montgomery, but was widowed in 1635 when her young husband died of smallpox.”
Upgrade image posted 6 June 2017 from karoline-von-manderscheid.tumblr.com/post/72670526235/circle-of-sir-anthony-van-dyck-1599-1641; filled in shadows, increased exposure and doubled size.
The staff at van Dyck's studio produced this copy of the last image. Her jewels appear to include diamonds.
Keywords: 1640, van Dyck, Mary Villiers, Mary Stewart, Mary Stuart, Duchess of Richmond and Lennox, Lady Herbert, Duchess, British, Villiers family, Stuart family, curly coiffure, hair jewelry, earrings, necklace, fitted bodice, U-neckline, brooch, clasps, elbow length puffed sleeves, cuffs, full skirt, wrap