I've seen stones like this a time or two in the past, usually sold as razor hones or in a lot with razor stuff and listed as coticules because for a lot of eBay sellers, 2-sided nat = coticule. Never won them, so can't speak from experience. Just from looks, highly doubt it's a Thuri.
Lighter side reminds me a little bit of the "Welch" razor hone I've had twice now. Mid-range to just barely passable finisher... but both of mine were glued to wood, not another stone.
I thuri would not be layered like that and is usually 1 piece and you will be able to see saw marks where cut....to me it looks like a coticule just a really dirty one. Might wanna raise a slurry on it and see of the color changes a bit or give it an overnight soak in simple green and see if the dirt raises off. The side shot with what looks like woodgrain is the side that tipped me off to coti. Im sure others here with stone experience will help you out further.
Yep that really looks like a mottled thuri-Schwedenstein. There are a few examples that Schwedenstein hones where glued to other stones to have a two-sided different fine honing stone.
Regards Peter