There is no shortage of Sada Yairi nylon strings on Yahoo Japan, and I bought my share while living in Japan. Plan was, to have a Sada from each year of the 1960s, but that was never fulfilled. I did have quite a few though, earliest was a 1961 with figured mahogany back and sides. The only Sada I kept and brought back to Canada is a 1971 rosewood. I saw several types of Yairi labels in Japan, close to 10 or so different ones. While I was there, precise info on the Yairis was very difficult to come by, even for the Japanese people who were members of web sites there. I heard Sada and Kazuo were cousins, brothers or uncle and nephew. One of the Japan Vintage books I have has an entire interview with Kazuo which may or may not reveal some truth but I can't read enough Japanese to know for sure, and I was told he did not like to speak of the connection between the two. Members here may be surprised how little some old Sada nylon strings can be had for in Japan, I know some members on some sites I joined when I lived in Japan did not believe me when I posted new guitars and how little I had paid for them. Along with the different labels I saw there were also several headstock shapes. My 1971 has a hand written label, some I owned in Japan had the blue labels and all were dated, leaving no doubt as to the years they were built. Never saw a Sadao that was of poor quality, each one was top notch. I would be wary of the new S. Yairis, the ones I saw were MIC and/or not nearly as well made as the old ones.
My primary classical for years was a stellar Tama tc15 cedar. It is old in the tooth and has been refinished French polished, but it still blows the doors off most of today's classics until you get up over 7 to 10,000 Canadian and order a new fangled double top classical cannon guitar. But I know that some of the old Yairi's were almost as good as the design used by Kohno who built the Tama production classicals.
Who actually built the incredible legendary Tama steel strings that really ticked off Martin and Co is another matter because they to could be incredible and consistently blew the doors of factory D18s hands down!!!!. Just maybe people trained by Kazou and Sadoa??? and his school of builders, who knows for certain. But the great Tama steel string guitars did disappear from history quite suddenly for some very strange reason or other which I cannot fathom... Guess I must be getting stupid in my old age LOL Might need to troll the Japanese web sites to see if I can pick one up on the sly. I dread the thought not arff arfff... Dreads are not really designed for anything other than chord play melody and single note chord melody and are useless for much else because of the skinny neck. IMO
If I ever did by a dread I would much rather play it with a electric wheel bow than fingerstyle. It would be easier and sound better I have no doubt. As far as dreads like D18's and the like.. this is a much better use.
Notice that this magic below is certainly not being done on a DREAD more a little like a wider neck with almost classical body shape, OM size and under with a wider neck is the sweet spot in steel strings always has been and is why I moved away from them cause you couldn't get a decent wider neck steel unless you had one made and that really ticked me off with the guitar industry in North America general.
But back to Yairi and how good they really were back in the late 1960's and into the 70's. Sadoa was sometimes better at making spruce topped instruments that really sang than Mr Kohno and everyone who played them frequently knew this could be the case.
With a good spruce top it is more of a crap shoot for the luthier to say the very least, if you don't tune the bracing chances are it will wind up sounding like a factory Larivee classical, which is OK but not stellar. Well built cedar top guitars always afford more sound but do not improve as much over time as do spruce guitars. The very same thing goes for steel string guitars. If a spruce top steel string has sweet even sound with great sound as you play up the g string and d string up neck right off the bat, give it a few years it can become really, really good. Cedar top classical and steel strings pretty much what you play is what you get.
Some older guitars with a very heavy plastic finish can improve with refinish, but by and large the early Yairis were finished very thin and refinish with french polish would not help them open up more. So like all things guitar you cannot know for certain until you actually play it what it will sound like in your hands.