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Have you talked to actual sellers of stones not via email, but rather in person??

My question to you is do you know Japanese or are you using Google translator. Also have you been to Japan and talk to wholesalers and other experienced people about stones?

I do not think the kanji means Mejiro, and koma is not the finest nagura. Naguras vary in quality just as any other Jnat.
This piece could very well be a Chu nagura, which is very common