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Jimbo take a look at this one
it is listed as Oohira, but same label as the stone the OP has

77974

I have seen Oozuko with the same label.
All that apart, if the stone works for it's intended...

One thing to consider Narutaki is a common denomination or stones coming from the region when the actual mine of origin is not known.
That really does not mater when the stone does the job it was...