The wagon (和琴) is a six string zither thought to be one of the few indigenous instruments of Japan. The instrument can be seen in prehistoric Japanese figurines, and differs from other chordophones brought to Japan from mainland China and Korea - the movable bridges, for example, are unique in that each is a joint from a maple branch. While the ancient tuning structure of the wagon is largely unknown, several tunings have been in common practice since the Nara period, each made up of two ascending 3-note chords (unlike most other Asian zithers that ascend in scalar patterns).
The Shinto musical style called Azuma Asobi is one of the ancient song dances (うたまい）associated with the eastern regions of the island (Azuma Asobi translates to “musical entertainment of the east,” and is likely associated with eastern animistic ceremonial song and dance of aboriginal peoples in the Yamato period [250 - 710]). Azuma Asobi is a ritualistic music and dance that is traditionally intended to scare away evil spirits, summon a deity, or bring protection, and is documented as a common festival Shinto festival performance since the 9th century. The traditional instrumentation is male chorus, lead male singer who also plays shakubyoshi, hichiriki, wagon, and a low pitch bamboo flute called the chukwan, though since the Meiji restoration this instrument is obsolete and has been replaced with the Korean koma-bue.
The ensemble configuration I've been researching for my current project is largely influenced by the traditional Azuma Asobi instrumentation, though rather than vocals I will use sho, the bamboo reed organ common in Gagaku (ancient Japanese music of the imperial court).
Research thus far has been focused on the wagon - reading, experimenting with tunings, studying a variety of scores, and improvising. Kyoto City University of the Arts has a beautiful instrument that I’ve had access to, which has been a huge help.
Traditional tunings for wagon are below, Azuma Asobi included (middle):
Score examples from various eras: