It was because Yoshimune was examining records of each family for revival of old style kyuba such as jarai (shooting) and inuoumono (dog-hunting event, a skill of an archery) and Mochihiro was given words 'those books are rare in the world and to be stashed away for many years.'
これは吉宗が射礼や犬追物など弓馬の古式の復興に熱心で諸家の記録を調べていたためで、「世に稀なる書ゆえ永く秘蔵すべき」旨の言葉があったという。