This all sounds logical. Would they make scythe stones in a 800(est.) grit rating? I’d have thought garden/farm implements topped out at about 400 grit to sharpen.Any old stone like that, that's been found in the US is vanishingly unlikely to be either Pyrenees/Saurat or Wastikivi. The production of those stones is tiny and before the internet - only for very local markets.
There are a large number of American stones that are the same type of rock, and look and act quite similarly; hard, coarse, dark grey and glittery. Which historically would've been sold as scythestones or grindstones. And you can probably double down on the certainty of it being American because you're in MA, and the north-east was the centre US scythestone quarrying and production. It's why Pike were founded and based in New Hampshire.
So yep - it's a coarser American stone, and probably quarried quite close to where you are.
Wastikivis are actually a fair bit finer than that, something like 2-3k, though you're right that these kinds of stones can often look a fair bit coarser than they actually are. It's because they're phyllites and schists, and much of the twinkly appearance is from the formation of larger micas, which are softer than silica and steel, and so don't affect the sharpening.