Below is a map of Samcheok (三陟 - 삼척) and Ulleungdo (鬱陵島 - 울릉도), from the 1893 atlas "Yeoljae Chwalyo" (輿載撮要 - 여재촬요). The map shows that Ulleungdo was associated with Samcheok in 1893. The map also shows a single island off Ulleungdo's east shore labelled Usan (于山). Usan was the old Korean name for Ulleungdo's neighboring island of Jukdo (竹島), which is 2 kilometers off Ulleungdo's east shore.
The map is important because it shows that Ulleungdo's neighboring island of Jukdo was still being referred to as Usando in 1893, ten years after Ulleungdo was opened to settlement.
Below is a similar map, which was made sometime between 1888 and 1894.
Thanks, GTOMR, for telling me about the 1893 map.