Thank you all for your replies.

Now that I think about it, they stone must have came to Bulgaria before 1950, perhaps long before that. It doesn't seem very plausable that in those years, in a small town in Bulgaria, a Japanese or Chinese stone appeared. Back then, we didn't have developed trade relationships with those countries. It seems more realistic that the stone came from Europe. Maybe the guess about Thuris is the closest one to the truth.

By the way, I used the stone on one of my razors that has very tough steel. It has really good tactical sense of honing. It works quite fast. It gives very good and soft finish (I do not use strop and paste). I am very pleased from the result!

In the end, the result is more important than the stone itself. But, it's better if a man knows his instruments.