these are very traditional flutes that are endblown on a slant-cut edge and tuned by adjusting the inside bore dimensions. A real one has very constrained dimensions and node placement. I make a loose approximation of a shakuhachi.
There is a variant called Hochiku, or as some write, Hocchiku that is closer to what I make.
Tom Deaver on Hochiku vs. Shakuhachi
attached below is a pdf of many notes on Shakuhachi design from