From what I've read the arm braces cover short barreled rifles and pistols. I've never seen anything that addresses the Shockwave or TAC 14.
If you look at the recent ruling, there are a few examples posted. A lot of the images of firearms in the ruling… they were from prior ATF approvals. They state as much on page 6484 (if you download the 98 page PDF… end of 7). As of 2014… ATF approved the setup and plenty were sold with braces.
Micro Black Aces Shotgun! Is it Legal?
Black Aces would include a copy of the ATF approval letter with their firearms.
And when ATF first changed their mind…
ATF: Some Black Aces Tactical Guns Are Unregistered NFA Weapons
I'm sure if you look hard enough, you can find the original letters online. I never owned one of those Black Aces guns, but had a brace on my TAC-14 for years until they changed the rules.