Can someone please help me identify this sharpening stone. I got this old stone from a friend at a very old house. He figures it belonged to his dad but did not have any need for it. I am trying to figure out what it is.

When I got it it was black so I boiled it in a pot with dishwashing detergent. The black stuff came out into the water almost instantly. This got it quite clean. I have also lapped it with some wet/dry silicon carbide (I think) sandpaper on a sheet of glass. It was very close to flat and now it is flat.

Can anyone identify from the photos what type of stone it is?

I appreciate the help.
Thanks