If you just google Yairi Gakki you get this article:
Sadao or Sada Yairi was a guitar maker and guitar manufacturer from Nagoya, Japan who was active from the 1960s to the 1990s. He sold guitars under a number of labels including Sadao Yairi, Yairi Gakki, S. Yairi, and Sada Guitar. He also manufactured guitars for B&M (distributed in the UK) Lowden, Sherry-Brener and a few guitars for Alvarez of St. Louis Music during the early 1970s. During the early 1990s, he had a line of handmade guitars sold by Samick in S. Korea.
Information on Sadao Yairi is scant and a number of Japanese Sadao Yairi collectors confirm that Sadao Yairi’s guitar making history is shrouded in mystery especially after a 1990s statement by Kazuo Yairi, of Alvarez Yairi fame, that Kazuo and Sadao have no working relationship.
The consensus is that Sadao and Kazuo Yairi are cousins who learned guitar making from Kazuo Yairi‘s dad, also named Sadao Yairi. The older Sadao, had worked as a guitar and violin maker at the Suzuki, Takehuru factory in Nagoya and then later opened his own business, Yairi & Son, which operated until about 1970-71. Nagoya is the major industrial port city in Aichee prefecture and one of Japan's long established guitar making centers.
In the late 1960s Sada and Kazuo went their own way and began making guitars under their own label, with Sada establishing S. Yairi guitars in Nagoya and Kazuo Yairi starting up K. Yairi Guitars in Kani, which is about 30 minutes from Nagoya. Both Sadao and Kazuo made guitars for Alvarez/St. Louis Music Company in the early 1970s, but Kazuo ended up getting an exclusive contract with St. Louis Music. Kazuo's guitars sold by SLM were first known as Alvarez by Kazuo Yairi and then Alvarez Yairi guitars.
In the early 1970s, Sadao had a number of guitar ventures including Yairi Gakki and S Yairi Guitars (gakki is the Japanese word for guitar). S. Yairi made guitars for B&M in the UK and for Lowden in the USA. Sadao also made some Alvarez guitars for SLM in the early 1970s. He went bankrupt in the late 1980s. As a result of the bankruptcy, he went to South Korea in the early 1990s and worked for Samick guitars, developing a short-lived line of handmade guitars labeled "Samick by S. Yairi".
In the 1990s Kazuo Yairi published a statement that there was no working relationship between him and his cousin Sadao (probably because of the bankruptcy). I believe that Sadao Yairi passed away during the 1990s.
Sometime during the late 1990s/early 2000s, a Japanese venture started to manufacture low quality guitars using the S. Yairi name. These guitars are made in China and are not the same quality as S. Yairi guitars made during the 1960s to 1980s.