Glanrafon Natural Sharpening Stone

Please study photo's, I am no expert on stones but have sold a few hundred over the years and as far as I can tell this is from the Glanrafon slate stone mine, has lovely marking on this which looks to me a lot like what I think it is.

Around 7 7/8" x 2 3/32" x 3/4"

Around 550g.

Will need to be lapped etc, the slighest dip when held with a rule up to light.