Hello Eod7,
I have a stone with the same stamp (see picture).
I asked Mr. Teshiba-san what is written on it, and got following answer:
"It is written 三河本名倉砥 in stone of the image.
三河 is a place name.
本名倉砥 is a real Nagura grindstone.
Mountain is different from the Mikawa Naguras."