Simply says that it's a top grade hone and again with the Nagura, you may notice a similar kanji on your Asano Nagura before sen- it's not asano, but it's also trying to say it's a top selection. The rest of the kanji says "lap me off, nice and flat if you want to know anything about how I hone" jk.
It's a nice looking toishi, I am always a bit leary of stamps. Most stone dealers have them and most the best stones don't have stamps, though plenty of great ones have them too, or there wouldn't be any motivation to stamp stones. The stamps and size suggest it was being marketed for razors, only metal can test its meddle.

What's the hardness? 4+? The chips make it look harder, but you say the slurry was grey? You might want to get yourself a tomo out of nakayama or other Lv5 or 5+ stone with soft particles, or a DN. I wouldn't expect that nagura to perform for finishing and until you slough off that outer layer of oxidation and dirt and grease on your stone, you won't be enjoying the true qualities of that toishi.

Looks like a nice stone.