Is a knoxx adjustable shotgun stock illegal in California?
For a pump shotgun? As long as it meets the minimum length requirement, no, its not illegal at all.
For a auto-loader? In California in order for it to be an assault weapon it has to have both a pistol grip and a folding or collapsible stock. Because I think the stock you are talking about has a pistol grip and is collapsible, it would probably be illegal for a semi-automatic shotgun.
“Knoxx Recoil Shotgun Stocks Suck” by Nutnfancy