The following map is entitled "Map of the Original Regions of Silla, Goguryeo, & Baeje" (新羅高勾麗百濟肇造區域之圖) and is from an atlas entitled "Dongyeo Bigo (東輿備攷)," which is believed to have been compiled sometime in the mid to late 1600s. The map below is one of thirty-two maps in the atlas, which is believed to have been made to aid in the understanding of "Dongguk Yeoji Seungram" (東國輿地勝覽) . The atlas is stored at Daeseong Hermitage (大成庵) in the South Gyeongsang Province city of Yangsan (梁山). The map is very similar to THIS MAP.
The map shows an island off the east coast labeled "Usan-guk - Now Ulleungdo" (羽山國 今 欝陵島). This is evidence that Usan-guk was just a reference to Ulleungdo and did not include "Dokdo" (Liancourt Rocks).