Grade-Indicator., patent, January 7, 1919; (https://texashistory.unt.edu/ark:/67531/metapth1259405/m1/1/: accessed June 22, 2024), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu.; .
Grade-Indicator., patent, January 7, 1919; (https://texashistory.unt.edu/ark:/67531/metapth1259405/m1/1/: accessed June 22, 2024), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu.; .