Suehiro Nagura stone.
This stone is not for sharpening.
Itis used to create an abrasive paste on the surface of the whetstone to speed up the sharpening and polishing process. It has a #8000 grit.
It is also used to flatten the whetstone and fix up some small damages. Suitable for use with waterstones up to #8000 grit.

Size : 70 mm x 22 mm x 19 mm (2.8″ x 0.9″ x 0.8″)
Weight : about 50 grams

Made in Japan.