if you live in MA, you need a preban underfolder in order to let it "under fold."
If you live in MA, and don't care about it "folding," then you can "pin" the stock so it's permanently "fixed" in a "deployed" positions.
Nothing too terrible. Not too many "definitions" out there that tell you how it needs to be fixed. Just so it's "permanent".... whatever the hell that means.
I live in a hyper ban state. And I own a underfolder. It's "fixed" but it also doesn't even run. Because I built it while I was blackout hammer drunk. Technically, it isn't even a firearm right now.