The decipherment of the monogram is not clear, though “C M” appears to be the correct reading.  The most likely candidate is Lady Mary Montagu (1711-1775), who in 1730 married George Brudenell, 4th Earl of Cardigan (1712-1790), styled Lord Brudenell until 1732, and known as The Earl of Cardigan between 1732 and 1766.  Mary Montagu was the daughter of John Montagu, 2nd Duke of Montagu (1^st creation), but as none of Montagu's sons survived him, his titles became extinct upon his death in 1749. His estates were inherited by Mary, whose husband, George Brudenell assumed the name and arms of Montagu. In 1766 he was created 1^st Duke of Montagu (second creation).