1772 Katherine Clayton, Lady Howard de Walden as the Persian Sibyl by Benjamin West (Audley End House - Saffron Walden, Essex UK)
Her article in thepeerage is here.
From bbc.co.
Benjamin painted two portraits, one posthumous portrait of Anna Maria Shutz (as the Cumaean Sibyl) and this one, in 1772, both dressed in turbans and exotic dress. If you look up Shutz, you won't find Anna Maria, but the next family listing is for Maria Schriver's husband and family, ex-Governor Shwarzenagger).
Keywords: 1772, West - Benjamin, turban, scoop décolletage, modesty piece, bodice, long puffed under-sleeves, clasps, wrap