The year 1904 was four years after the Korea's Imperial Edict #41, which mentioned Seokdo, and one year before the incorporation of Takeshima (Liancourt Rocks) into Shimane prefecture of Japan.
Although it may be a little hard to identify the colour of the islands in the Japanese Sea, all the islands including "Taka I", "Dagelet I", "Matsu I (with Boussole Rocks)", "Liancourt Is. (To France)" and "Hornet I. (To England)" were coloured in pink (horizontal pink line) - the same as Japan's mainland. (Korea was coloured in light purple with longitudinal and horizontal lines)
You may notice that there are two Ulleungdos (Dagelet I and Matsu I) and two Liancourt Rcoks (Liancourt I and Hornet I). Although the information was inaccurate, this map clearly shows that USA didn't think that Liancourt Rocks to be Korean territory even after the 1900 Imperial Edict and before the incorporation of Takeshima (Liancourt Rocks).
Please notice the colour of the islands including Liancourt Rcoks (pink horizontal line) which is the same as Japan (left), and different from Korea (right; purple longitudinal and horizontal crossing lines).