[Letter from Mark F. Heller to Isaac H. and Daniel W. Kempner, February 7, 1956], letter, February 7, 1956; (https://texashistory.unt.edu/ark:/67531/metapth1361025/m1/1/: accessed July 11, 2024), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu.; .