Thanks Michiel. I found somewhat of an answer: http://forum.nassrasur.com/showtopic.php?threadid=7819
(Hope your German is good enough.)
Apparently these are stones from a mine a couple of kilometers from the original Thuringen mines. They are not sold by Muller as Thuringen stones, but apparently they are sold as such (wrongly) on the website I mentioned. People who used these stones describe them as mediocre, although one person was quite positive.
What do you mean by "I call BS sadly"? And whose honing advice is wrong?