In his 1186 letter addressed to Tsunefusa YOSHIDA, Kanto moshitsugi (court-appointed liaison with the Kamakura bakufu), Yoritomo said that he would autograph his Kao (written seal mark) for migyosho that were not written by OE no Hiromoto or TAIRA no Moritoki (to guarantee their authenticity).
文治2年（1186年）に頼朝が関東申次吉田経房に充てた手紙には、「大江広元か平盛時の筆跡ではない御教書には自分が花押を据える（有効性を保障する）」と記した。