Kinenhin - 記念品
These are Souvenir Nagura. You can see the extra stamp on the side adjacent to the typical quality stamp.
Stones with this mark are a keep-sake, or a memento of visting the Aichi mine where Mikawa Shiro Nagura are quarried. Pictured here is a full set of 3, Botan, Tenjyou, and Mejiro. These were made for me by special order. Typically, you would only see one piece at a time.
