Old stock nagura mined in Aichi prefecture the 1960’s. These are great for use with hard fine stones to either make a more aggressive initial slurry to act as a pre-polishing medium for fine finishing and to slightly rough up the surface a hard stone that is becoming glazed. By exposing fresh abrasive the stone will cut faster. Typically these are used on stones for sharpening razors but can also be used with hard stones for extra fine finishing on knives whose use and steel accommodates this fine a finish.
We do not have the specific strata on these stones, they are only marked “Mikawa shiro nagura”