The lid is near Ryūga Cave, one of the longest limestone caves in Japan. The figure in the center of the cover is "Ryu-kun" created by Takashi Yanase and associated with the cave. You can also find an illustration of an urn found in the excavations conducted in the cave, and the cherry blossom.
Translation of the text on the cover: 龍河洞リューくん - Ryugado Ryu-kun やなせたかし - Takashi Yanase かみし おすい のうしゅう- Kamishi Osui no Shuu - Sewage of a farming village