The term Gensui, which was used for both the Imperial Japanese Army and the Imperial Japanese Navy, was at first a rank held by Saigō Takamori as the Commander of the Armies/陸軍元帥/Rikugun-gensui in 1872. However, in May 1873 Saigō was "demoted" to general, with gensui thereafter no longer a rank as such Gensui will return only in 1898 but not as a rank but as additional honorific title to the rank of general or admiral.
Imperial Rescript No. 5 on January 19, 1898 introduces the title of marshal.
第一条 元帥府ニ列セラルル陸海軍大将ニハ特ニ元帥ノ称号ヲ賜フ
第二条 元帥府ハ軍事上ニ於テ最高顧問トス
第三条 元帥ハ勅ヲ奉シ陸海軍ノ検閲ヲ行フコトアルヘシ
第四条 元帥ニハ副官トシテ佐尉官各一人ヲ附属セシム
Imperial Rescript No. 96 on May 24, 1898 introduces the marshal badge.
Published on May 25, 1898.
Full color photos of original Rescript could be found here https://asiamedals.info/threads/ori...no-96-signed-by-emperor-on-may-24-1898.23025/