When Rose Selfridge meets Roderick Temple at the National Gallery, she fibs and tells him her last name is Buckingham. In fact, the real Rosalie Selfridge was named Buckingham before her marriage to Harry Gordon Selfridge. The Buckingham family was a very prominent one in Rosalie's native Chicago, and relatives of Rosalie gave the funds for the enormous Buckingham Fountain in Grant Park, which is still one of Chicago's best-known public landmarks.
When Mr. Selfridge says "nice hat, son" to Gordon in season one, the actual line was meant to be "nice Toby, son," but the young actor didn't understand the reference and so the dialogue was changed.
In earlier seasons, the Selfridges have four children. Gordon Selfridge (Adam Wilson), Rosalie Selfridge (Poppy Lee Friar), Violette Selfridge (Freya Wilson), and Beatrice Selfridge (Raffey Cassidy). In order to show proper age progression in later seasons, actors were replaced. Gordon Selfridge was now played by Greg Austin, Rosalie & Violette were played by real life sisters Kara & Hannah Tointon. Beatrice is not mentioned at all in season 3, even though it seems all of the Selfridge family is now residing back in London.