In 1863, two years after he made the "Daedong Yeojido," Kim Jeong-ho wrote a geography text, called "Daedongjiji (大東地志), in which he described Ulleungdo. In his description, he wrote that Ulleungdo was once called "Usan" (于山), and then he went on to describe the various features of the island, including that it had three towering peaks, seven streams, five to six bamboo groves, dozens of remains of [old] dwelling sites, old boat houses, and stone-piled graves. He also mentioned Jeojeon-dong (楮田洞), Gong-am (孔巖 – "Hole Rock"), Jutogul (朱土窟 – "Red Earth Cave"), and the "four or five small islands" off Ulleungdo's southern shore, but he did not mention Ulleungdo's most prominent island off its east shore, which his 1834 map labeled as "Usan" (于山). If you compare Mr. Kim's 1863 description of Ulleungdo with his 1861 map, you will notice that everything mentioned in his description appears on the map, which suggests to me that it was no mistake that he left the "island of Usan" (于山島) off his 1861 map.
I think that by 1861 Kim Jeong-ho may have come to the conclusion that Usando was not a neighboring island of Ulleungdo, but was just an old name for Ulleungdo, as he said in this 1863 description. Coincidentally, in 1882, Ulleungdo Inspector Lee Gyu-won (李奎遠) told King Kojong that Usando was just an old name for Ulleungdo, and that Ulleungdo had no neighboring island by that name. It is possible that at that time in Korean history that was the common belief, but that is only my guess.
The following is Kim Jeong-ho's 1863 description of Ulleungdo (minus the history) and his 1861 map. By the way, in Kim Jeong-ho's history of Ulleungdo, he did not mention An Yong-bok (安龍福), which may mean that he did not consider him an important figure in Ulleungdo history:
1861 Map of Ulleungdo (Click on the map for a much larger picture)
Ulleungdo is in the sea due east of this “hyeon” (縣) and is the old Usan 우산 (于山). Other names are Muleung (武陵 - 무릉), Uleung (羽陵 - 우릉), and Uleung (芋陵 - 우릉). It has a circumference of about 200 ri, and the distance from east to west is about seventy ri, while the distance from north to south is about fifty ri. Three dangerously high peaks tower above the island and are pure rock. If you climb up to a high place on a clear day and look into the distance, it [Ulleungdo] looks like a shimmering cloud. With a fair wind, it can be reached in two days. The Japanese call it Takeshima (竹島 - 죽도) and it is close to Japan’s Oki district. Japanese boats occassionally come to fish (倭舡漁探者時到).
From the central peak, it is about thirty ri to the shore due east, forty ri due west, twenty ri due south, and twenty ri due north. The are six to seven streams, five to six bamboo forests, and dozens of [old] dwelling sites (居址). There is Jeojeon-dong (楮田洞), Gong-am (孔巖 – "Hole Rock"), Jutogul (朱土窟 – "Red Earth Cave"), “seokjang” (石葬 – stone-piled graves), old boat houses (古址船泊處), and “places to wait for fair wind” (待風所). On the south side of the island are four or five small islands. The center of the island is all deep valleys with streams and rock cliffs. There are many cats and rats that are so big they are unrecognizable. [狙 means “monkey,” but it was probably supposed to be 猫 (cat).] There are also runaways (避人). There are peaches, plums, mulberry, edible herbs, rare trees, and many strange, unknown plants.