Nichiren used the back side of Gyosei monjo (administrative written materials) which were used by the Chiba clan and given to Nichiren by Tsunenobu TOKI, a vassal of the Chiba clan, to write letters to his disciples and patrons of the temple.
千葉氏が用いた行政文書を、千葉氏の家臣富木常忍が日蓮に譲り、日蓮が弟子檀那への手紙などに使った。