Nope. That's not a stock. That's a "stabilizer". It's designed (wink, wink) to stabilize the "pistol" so you can accurately fire it one handed. If it was designed to fire two handed it would be illegal. No kidding. They come with a letter from the BATF (or whatever they call it these days) stating that they are legal.
So, if it is put to the shoulder, its illegal? Hadnt seen one of those before. This is like the silliness of using a bayonnet lug, or a pistol grip, or a barrel shroud etc. to turn a sport utility rifle into an “assault rifle.”