a. correspondence from E. T. Seton identifying himself and a friend in a picture from New Sources of Indian History, 1934.
b. correspondence with the Museum of Science and Industry (Chicago) regarding identification of the pistol used to kill Sitting Bull, 1944.
c. correspondence with Supt. Edward Luce, Little Bighorn Battlefield National Monument, 1952, regarding: 1. Campbell’s offer to sell a saddlebag from the Battle of the Little Bighorn River.
2. the identity of the killer of General George A. Custer. d. correspondence from Edward Luce regarding George A. Custer’s height, 1952.
e. correspondence with the Smithsonian Institution regarding the carbine used by Sitting Bull, 1952.
f. correspondence from Campbell to Life Magazine regarding the transfer of Sitting Bull’s body to South Dakota, 1953.
g. correspondence with the Kalamazoo Public Museum regarding Sitting Bull’s shield and other items, 1956.
h. general research correspondence, 1929-1957.