The top mark reads Izumo. The bottom is the "Dai" in a circle mark. Both marks are usually found on these wares. Sometimes joint mark with Izumo Jakuzan (出雲若山) is found. Originally the reading was Izumo Wakayama, but it has been confirmed by Anri Sasaki, a Japanese curator at the Tezen Museum in Shimane, that the reading is Jakuzan. It appears that people confused the Izumo Wakayama pottery with Wakayama, the prefecture. The kanji characters are different but the sound is the same. Because of this confusion they changed the name using another reading of the same kanji characters. I believe this change may has happened around the turn of the 20th century. Thanks to my friend Alice Gordenker for verifying this information.
Reference works of the time Morse and Brinkley conflict. Morse used Wakayama and Brinkley used Jyakuzan. Morse published his work in 1880, while Brinkley published in 1901. This is how I surmised that the change must have occurred around the late 19th or early 20th century.
