I'm wondering what type of flag is pictured in the link below. The 3rd Corps design was a diamond, but this appears to have more to it. If a Corps flag, what is the purpose of the rectangle in the center?

The caption on the website states, "The residence of Chief Quartermaster of the Third Army Corps, Brandy Station, Virginia, during the American Civil War, December 1863. Photo from 'Gardner's Photographic Sketch Book of the Civil War' by Alexander Gardner, vol. II, 1866. (Photo by James Gardner/Hulton Archive/Getty Images)"

Ed