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Check out this thread post 33 for photos of a Special Stone with measurements, it does look similar to yours.

Here

191344

The Special Stones that we talked about earlier, had that shiny metallic sheen, like yours and I think the size might match. Either way it is probably a nice slate finisher.
Be careful lapping on a...

The black one looks like slate, possibly a good one from the size, sawmarks and slurry.

The white one looks like possibly a hindostan from the photo, but may be an ark, or greek, though arks and...

Yes they are great Knife stone, will make you bride happy if you put her kitchen knives to it regularly.
Also as said, good for tools.

For razors there are faster solutions, though a hard Ark...

The last one is the most interesting and possibly the finest.

If it was purple or chocolate brown….

If it’s black black most probably a blue slate or maybe a Black Ark.

The slate may slurry...

So, what color is it?

One photo looks green the other purple.

Some close ups would help as would photos of the slurry.

So, how many hones are there?

The last photo is that a separate stone or is it a close up of the one that is mounted in the wood case.
That one looks interesting.

As far as grits go, no not...