When Isobel Crawley asks Violet Crawley how she's feeling, Violet replies, "Like Dr. Manette. If I don't get out of this house soon, I shall remember nothing but my number." This is a reference to Charles Dickens' novel, A Tale of Two Cities. In the book, Dr. Manette is wrongfully imprisoned for 20 years. As a result, he gets amnesia and remembers only his assigned prison number.
This is the second episode in a row where Hugh Bonneville has significantly less screen time due to his schedule. He was filming Monuments Men at the same time.