McCallum, Robert D. [Letter from Robert D. McCallum to Harris Leon Kempner, April 23, 1965], letter, April 23, 1965; (https://texashistory.unt.edu/ark:/67531/metapth1425443/m1/1/: accessed July 18, 2024), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; .