Hardness: 10
Fineness: 10
Cutting Strength: 2.5

Definitely for skilled sharpeners only. Impossible to use for a beginner uses.

The stamps read,

from the top right "Ukeai Jyun Shou-honyama (Guaranteed genuine Shou-honyama = Nakayama)",
bottom right "Yuu-ryou Shiage Toishi (High quality finishing stone)",
bottom center "Maruichi-jirushi Tokusen" (Circled-ichi character Specially Sellected)",
middle left "Kamisori" (For Razors)".

People used to say "If you want to get a good stone for reasonable price go visit, first a barber and after that a doctor, whose gone out of business." This is because barbers needed best of the best stones to sharpen their razors, and doctors to sharpen their scalpels. Since these tools are the hardest tools of all, stones chosen for razors tend to have strong cutting strength, thus good for sharpening hard alloys.

(Maruka) Shou-hon-yama is the best grade stone from Nakayama sold by the sole miner of this mountain, Hatanaka Natural Sharpening Stones Stone Industry. Any (well basically, but there are many that are not so good, so we have to test all of them before selling) Nakayama stone that has this stamp (unless it's a fake) is guaranteed of its highest quality. Maruichi Maruka stones were quarried before the mining was taken over by Hatanaka family, when the owner of the mine was Kato family. Maruka's (= circled "Ka") "Ka" is Kato's first character "Ka".