This was a fictional set-up that a historical novelist Ryotaro SHIBA created in his book 'Shinsengumi Keppuroku' (Record of Shinsengumi Bloodshed), which led to the image of Soji OKITA's favorite sword as Kikuichimonji.
これは歴史小説作家の司馬遼太郎が著書「新撰組血風録」の中に描いた架空の設定で、ここから沖田総司の愛刀は菊一文字というイメージが定着した。