State-designated Cultural Heritage No. 1289 (designated in December 1998), the letter of instruction sent to the Pyeongan-do military commander Yi Yun-son concerning the mobilization of troops, was personally sealed by King Sejo in March 1457 (the 3rd year of his reign).

Yi Yun-son was born in Hanyang (present-day Seoul) in October 1414 (the 14th year of King Taejong’s reign), and belonged to the Gapyeong Yun Clan.

In 1438, he passed the state examination for the recruitment of military officers and served as military commander of Gyeongsanjwa-do, governor of Gangwon-do, mayor of Hanseong, and Grade-3 official at the Finance Ministry among other posts.

This letter of instruction concerns the mobilization of troops by a provincial official. It was so important that the official bearer of the missive carried it in a special case. The letter is kept by Yi Yeong-ok, the 18th grandson of Yi Yun-son, residing at 326-9 Junghwa-dong.