He quotes the first four lines from an epitaph - "Should simple village rhymes attract thine eye, Stranger, as thoughtfully thou passest by, Know that there lies beneath this humble stone a child of colour haply not thine own" - as an example of "changing attitudes".
I can only assume this means that he thinks the word "haply" means "happily". It does not. Haply means "by chance", something that "happens".
Much is known - if not about Anna Maria herself - about her father, Gustavus Vassa. The rhyme goes on - "Her father born of Afric's sun-burnt race, Torn from his native field, ah foul disgrace; Through various toils, at length to Britain came espoused, so Heaven ordained, an English dame, And follow'd Christ; their hope two infants dear. But one, a hapless orphan, slumbers here."
Gustavus Vassa, whose real name was Olaudah Equiano, was born in Nigeria in the 18th century and sold to slave traders. Transported to Barbados, he was given the name of Gustavus Vassa, after the king of Sweden.
Olaudah travelled to Canada, the Arctic and the Mediterranean and, as a free man, settled in England. He played a significant part in the abolitionist movement and wrote The interesting narrative of the life of Olaudah Equiano, or Gustavus Vassa, the African. Anna Maria was the eldest daughter of his marriage with Susannah Cullen, from Soham, near Ely.