The identification proposed by the inscription on the cartellino must be erroneous as the portrait post-dates the death of the 1st Baron Chandos. The writing on the cartellino appears to be in an 18th century hand, while the left edge of the panel (including most of the coat-of-arms) is a later addition. If the sitter is a member of the Brydges family, he is more likely to be Giles Brydges, 3rd Baron Chandos (1548-1594).
The Duchess of Buckingham referred to in the label on the panel’s reverse is probably Catherine, Duchess of Buckingham and Normanby, née Lady Catherine Darnley (1680-1743), an illegitimate daughter of King James II and his mistress Catherine Sedley, Countess of Dorchester. She married John Sheffield, 1st Duke of Buckingham and Normanby (1648-1721) in 1705, and the portrait appears to have passed to her step-son Sir Charles Herbert Sheffield, 1st Baronet Sheffield (1704–1774), the son of his father’s mistress Frances Stewart.