From what I can find, it's Steward. From the quote I keep seeing about William Selby, Sr. arriving in MD on a ship as an indentured servant t Lieut. Truesman, a Mary Steward was with him on the ship in 1659. I found this on Mary: Married to Mary Steward, born 1639 died 9/5/1718 in Calvert Co., MD (Mary Sabrett formerly Steward aka Selby, born in Scotland). Wikitree has more info on her family. From Wikitree "William W Selby was born on September 16, 1630, in Badley, Suffolk, England. He was the child of James Selby (1605–1680) and Mary Armes (1615–1628). William married Mary Steward in 1680 in Prince George, Maryland. William passed away February 25, 1699, in Leith, Prince George's county, Maryland, at the age of 68."
So, Willie...erm William Selby, Sr., had a daughter named Mary, was married to a Mary and appears it mother was also Mary - Mary Armes, born in Worcester, England.. Source: https://www.wikitree.com/wiki/Selby-341
My grandmother was a Selby and I've traced her to William, Sr. :)