After they whistled as their signal to have Ako Roshi gather around by the entrance, they showed his head to three people who were guarding the front gate, and it became clear that it was Kozuke no Suke.
そして合図の笛を吹き後、玄関前に集合した赤穂浪士たちは表門番人の三人に吉良の首を見せて間違いなく上野介であることを確認した。