My opinion, if you start with a std. M4 stock and cut the butt off and attach a Tailhook, you have a stock with a brace on it. Which means you still have a stock on a firearm. Which the ATF says is an SBR.
I don't see how attaching a brace to a stock legally makes it a brace.
When I did my first pistol I thought about a Thordsen type setup and was going to cut an m4 stock with a CAA saddle down to just enough to support the saddle and disable the adjustment lever.
But while it's functionally the same as a Thordsen, cutting off part of a stock does not legally change it to something else.
At last count I have eight pistols with Shockwave or SB braces. So I love where all this is going, and I respect anyone who is willing to put a toe over the line to push the boundary. I just can't afford for that toe to be mine.