According to Wikipedia:

Omura Sumitada (????, 1533-June 23, 1587) Japanese daimyo lord of the Sengoku period. He achieved fame throughout the country for being the first of the daimyo to convert to Christianity following the arrival of the Jesuit missionaries in the mid-16th century. Following his baptism, he was known as "Don Batholomeo". Sumitada is also known as the lord who opened the port of Nagasaki to foreign trade.


(Wikipedia contributors, “Omur Sumitada," accessed 2/16/08)

This reference appears on page 193.
Robinson uses this reference when talking about a secret treaty that was arranged by Omura. He also describes how Umur invited the Chinese to land and trade at a fishing village and a harbor called Nagasaki, and to some extent he explains steps that were taken for preparations intended for future expeditions.