Zuisho is a sport of a regular JWP.
Zuisho and Kokonoe are two sports of JWP popular gor bonsai. They are pretty much the only ones that will air layer and ground layer pretty easily. They will also root from cuttings, which is also very rare! The needles are very similiar in appearance to your tree and Zuisho. Kokonoe has more rounded, grey buds in winter, while Zuisho is more pointed.
Kokonoe tends to bulk up its branches. That is they appear to thicken faster than Zuisho. I noticed your branches were thin, which is a good thing! So again, I was led to believe your tree may be Zuisho.
By the way, there are no yamadori Zuisho. If your tree is truely yamadori, then it's neither Zuisho nor Kokonoe.
In Japan, JWP are grafted onto JBP to force them to grow faster. As you say, JWP is a slow grower. They also vary in needle quality when grown as seedlings. They don't breed true. Often seedling needles will be long, or twisted, or unruly. That is, they look like bottle brushes rather than the nice tight tufts that look so nice and manicured.
Of course, the downside is JWP scions have the smooth grey "juvenile" bark for about 20 years before it finally sheds off and a more attractive flaky bark is revealed. Then it's another 20 years before it starts to form plates.
Now, about the grafts... The JWP to JBP grafts have been know to fail. Even when the tree is mature. Especially if someone is trying to get the tree to grow faster by heavily feeding it. The growth rates of the JBP stock and the JWP scion separate.
Especially on Zuisho and Kokonoe! So, the propagators in Japan are grafting Z and K onto JBP stock, with the intent of layering the JWP portion off at about age 20. By that time, the trunks have developed sufficiently to look pretty good. So now, the trees are growing on their own roots.
Now... Consider this... Would it have been possible 40 years ago for a seller to have represented a tree to a buyer as a "yamadori" since it was on its own roots when it actually had been started by grafts years before? Especially to a European tourist who wouldn't know it was possible to do such a thing???
I'm not saying that it was. But there's no way to know for sure at this point. Maybe a detailed examination of the underside of the rootball could determine it. Maybe.
Here are two JWP, on their own roots, that used to be grafted onto JBP stock. The JBP roots on both these trees have been removed. These trees were purchased from the seller before the JBP roots were removed. They both had the double root systems. And the JBP roots were removed by the buyer.
A Kokonoe:
And a Zuisho: