Ahn Yong-bok mispresented himself as a Korean official in 1696 to deceive the Japanese government (Shogunate). He went from Oki island to the mainland on their ship with a fake banner that claimed he was a Korean tax official. The banner say said, "Tax General of the Two Islands of Joseon's Ulleungdo (朝鬱), Vassal An on Board (朝鬱両島監税将 臣 安同知騎).
The following is from the book, “Special Chronicle of Inaba County (因府歴年大雑集), written by Okajima Masayoshi (1784-1859).
I do not know exactly why the flag of this ship was left, but it has been kept at the house of Chaya Heisuke in Aoya since he brought it back home. In the Chronicle of Inaba, it was written that the two islands of “朝欝” may have meant Ulleungdo, which we call Takeshima in Japan, and Jasando (Usando), which we call Matsushima in Japan. I find it is hard to believe this theory, however. since Takeshima and Ulleungdo were originally different islands. Takeshima is located offshore of Tongrae-hyeon in Gyeongsang Province (慶尚道東莱縣), and Ulleungdo is located in the sea off Myeongju (Gangneung) in Gangwon Province (江原道溟州). Its old name was Usanguk. I think the Korean official has been probably dispatched each year to the island to inspect whether the crops are good or bad and then was sent to Hakushu (of Japan) as an envoy. I think “朝” of “朝欝” must be an abbreviation of “朝鮮” (Chosun). I hope somebody with extensive knowledge will comment on this. By the way, Takeshima is an uninhibited and isolated island, and not suitable for farming so that you guess whether it [the theory] is right or not.
Notice that the author thought that Takeshima and Ulleungdo were different islands. Notice also that he believed Ulleungdo could be farmed, but Takeshima could not. He also said that Ulleungdo was east of Myeongju (Gangneung) in Gangwon Province, but said that Takeshima was farther south, offshore of Dongrae in Gyeongsang province. Though he seems to have understood the location of Ulleungdo, it seems that he may have thought Takeshima was Liancourt Rocks since he did not give an opinion on Matsushima and since Liancourt Rocks are southeast of Ulleungdo and unsuitable for farming.
Another interesting thing about the passage is that Mr. Okajima Masayoshi apparently had not heard that An Yong-bok was an impostor, which was most likely why An left his fake banner in Japan. He probably feared getting caught with it back in Korea.