It is hard to say exactly what each of these is without having each in hand. I was the USA distributor for MST Mueller in Germany. They offered NOS Thuringens and just one they called a waterstone. The Thuringens were either already cut stones from old stock or newly sawn from blocks quarried years agon in the Thuringen region.....the same place the Eschers came from. Escher of course is a brand name and dozens of brands out there all offered the same very fine grained blue/grey to green/grey stones. The NOS ones were 5" x 2 1/2" or so and came with a rubbing stone. I think I was getting 4135 to $175 depending on supply, the condition of both faces and whether I had lapped them.
The other stones were either 2" x 8" or 3" x 10" and were quite dark, almost a charcoal grey. They were originally offered to me as Thuringens but research by a friend local to MST revelled they were from the Hunsrueck region. These were very nice stones, cut well but are not Thuringens. They were a good deal less expensive.
That was years agon so no telling what the status is today.
Tony