The 80-string koto (or hachijugen) was an invention of Japanese composer Michio Miyagi which appeared In 1923. He added 67 strings to the traditional 13 string koto design, creating an instrument much like a western harp. Together, the 80 strings provide a far larger pitch range than the standard Koto. It is widely seen as a short-lived experimental instrument.
80-string koto Wikipedia
The 80-string koto was largely hand-constructed as only limited machinery existed to manufacture it. It was built along similar lines to the common Koto, but with reinforced design elements. The platform where strings are tied runs the entire length and breadth of the instrument to accommodate the increased strain.
As with other kotos, The kiri wood (paulownia tomentosa) is molded and treated. Modern bridges are often made of plastic and they can be found in both small and large sizes. These bridges are rearranged as needed during playing. Strings are traditionally made from Silk threads, although plastic nylon strings are often used as a cheaper alternative.
The bridges are arranged according to the particular tuning used.
This kind of koto is not widely used. There are few extant examples of the instrument today, as it never achieved the popularity of the 17-string koto, Miyagi's more widely accepted invention.