Masumichi Kato's 1950s-60s old Nakayama stock.
This is a super fast and fine-cutting stone. Flawed, yes, but worthy, you bet ya.
This is a tool-size stone 205 x 79 x 40mm thick that will dominate all irons and many types of steel including those used for straight razors.
The missing corner and the angular inclusions were worked around by a master sawyer to preserve the most while leaving the fewest alternate grit lines to the side and back. The main front inclusion is partially filled in with manganese and this does not scratch steel. On the back of the stone is a toxic inclusion that whips up along the edge where it peeters out and disappears before it reaches the top. All of these lines can be picked out with a stout needle or scribe when needed.
The #15 is a middle grade from the Nakayama mine. A photo of this stone appears on page 176 in my book "Japanese Sharpening Stone Heaven-Narutaki Toishiyama"
#299 Nakayama odd shaped bench stone
All of my stones come with a tested and sized tomonagura at no extra cost to you.