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Omura Sumitada (大村純忠, 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, "Lord Omura Sumitada,")

This reference is frist found on page 193 in The Years of Rice and Salt.
Omura is a real historical reference that Robinson uses to keep his alternate history realistic. Omura is important because he signs a treaty to secretly allow the Chinese to lead an expedition near Nagasaki.