Asano stamped Jyun Mikawa Naguras are natural stones only found the Kitashitara district of Aichi Prefecture. They can be used to hone razors, where the smaller ones usually or serve as flattening and mud creating stones for larger sythnetic / natural sharpening stones.
They can be further classified into 4 grades (grits):
Botan, Tenjyou, Mejiro and Koma, where the latter is finder than the former.
The mine was closed in Japan back in 1976 and all the new naguras are just old stock, as a result genuine Asano Naguras, particular bigger ones, are now very hard to fine. K&S only carry geniune Asano Naguras from one of the biggest suppliers in Japan.