Oh joy! A double grafted tree! A JBP seedling, grafted with Thunderhead, grafted with JBP seeding foliage!I've seen some Thunderhead with really really good looking trunks and nebari at local nurseries. I suppose they could be used to graft normal Japanese black pine back on to them.
There is a JBP on the .99 Cent Bonsai Auction on FB.
This is the only resource I could find:Mikawa are cork bark JBP right?
I would avoid those, they are rather finicky and not as vigorous as regular JBP