Thanks Richard!
It's a confusing stone to be sure... I put baking powder on it as instructed and there was no notable reaction.
Put one of my go-to razors to it this evening. I have a couple razors that I consider stupid-proof and seem to take an edge well off any good stone the first time. I can report that this stone provided a pretty fine edge. I would say it's pretty similar to my larger charnley forest and very similar light delicate feedback, but notably faster. It doesn't have the "clay" sort of smell of a cf, so that's out. also of note is that the old box that it came in was definitely hand made, and there was no trace of oil. So this is and always has been used as a water hone it seems.
I should mention that it's slightly more green than the images appear. It also seems just slightly lighter than what I would expect from a slate stone of similar size. That could just be my mind playing tricks though, I havent gone through the steps to verify this.
If anyone has any more ideas of what i may be based on that, please let me know. would love to put a name to this little fella