Nice little coticule there, my bet would be an older mined coticule due to the discoloration on the sides and edges.

Older mined coticules were mostly attached to Belgian Blue rock; not necessarily hone worthy, as honing on Belgian Blue Whetstone is a relatively modern thing and they used to back up coticule with whatever Blue rock they had at hand.

But this looks quite dark for BBW, rather like slate, as mentioned; so I'm guessing this is a homemade solution.

What size is it, anyway? It seems very much to be around 30 X 100-120 mm, which was quite a standard size for small Thüringer hones, if I'm not mistaken.

Keep in mind, this is very much spitballing though, as it's hard to say anything other than 'Yes, the creamy side is most likely a coticule.'

Personally though, I would definitely test both sides, as you maybe (just maybe) have a homemade coticule-Thüringer combination. Which would be totally awesome. I mean, what's the worst that could happen?

I would test it by going through your regular progression, then add one layer of electrical tape (to make sure the edge is all test hone) and do 30-50 light X-strokes on water only.