Dahlia Lithwick of Slate, in a typical treatment that also attacks Dixie Outfitters, refers to the Confederate flag as “the proud symbol of the Ku Klux Klan.” In fact, the KKK — a nationalist, nativist outfit if there ever was one — used the US flag exclusively in its days of power.
The 1965 book by David M. Chalmers, Hooded Americanism: The First Century of the Ku Klux Klan, 1865-1965 (Garden City, NY: Doubleday & Co., 1965) was the definitive treatment at the time. It features many pictures of the Klan in operation. Not one shows a Confederate flag. Many feature the US flag.
The Klan in Tulsa, 1923
Klan Marching, 1925
Klan in Nation’s Capital
Klan in 1960s Birmingham, Alabama