These are musical instruments (Bonongu) used in a Buddhist memorial service together with waniguchi and kei gongs. They are all percussion instruments and have rings(Chu) on the shoulders for strings to hang them.
Waniguchi is a flat, round instrument with a hollow body. It is called waniguchi ("alligator mouth"), because the slit made on the bottom half of the side looks like a mouth of an alligator. This waniguchi is cast in bronze with a plum flower-shaped hitting area in the center of each surface, which is surrounded with an arabesque pattern. On one surface, there is an inscription that says that this was dedicated by a court official to Gokurakuji Temple in 1001 (「極楽寺長保三年辛丑」「願主信判官代高向朝臣弘」).
Kei is originally a Chinese musical instrument made of stone or gem as the Chinese character (磬) shows. It is in the shape of the Japanese kana "へ (`he')" and used to be sounded while hanging more than one kei in a row. This instrument was later incorporated into Buddhism. Although most kei shifted their shape from "へ" to a chevron after the Heian period, some are in the shape of a lotus flower or a butterfly. This kei has a rare shape of a butterfly with its wings spread out. It is cast in bronze with a plum flower-shaped hitting area in the center of each surface, which is surrounded with an arabesque pattern. While it seems that there used to be rings on the shoulders for strings to hang it, they have been lost and only traces remain now.
These waniguchi and kei were excavated together with door clamps from Joyama in Miyabuchi, Matsumoto, Nagano in 1939. Since they both have the same plum flower-shaped hitting area and arabesque pattern, it seems that they were produced as Bonongu of Gokurakuji Temple at the same time in 1001. Both of them constitute valuable materials since they are the oldest existing instruments with an inscription of the creation year. As a set they are designated as an important cultural property.
The door clamps, which were excavated at the same time, are designated as an accompaniment to the instruments.