I have one. Haven't shot the sks with it yet. I've never seen a manufacturer name. Seems they drift in from the ether.
I had to do some sanding at the back by the dust cover and had to glue a small block of wood at the front of the internal mag to make the fitment work.
We've been wondering about these for many years. Not sure we've ever gotten the story. I wonder if these ones being sold are actually take-offs. I'm pretty sure North Sylva imported a bunch of rifles fitted with them about 5 years back and I don't think they sold that well. Probably a Minelli stock.