jion kanazukai; historical kana usage for kanji on-yomi
A system of historical kana spelling applied specifically to Sino-Japanese readings (on-yomi) of kanji, as opposed to native Japanese readings. Used in classical texts and older dictionaries. The usage note gives the example カゥトゥ for 高等.
In jion kanazukai, 高等 may be written as カゥトゥ.