Mark Barnes, Esq., outside counsel to SB Tactical, makers of what is commonly referred to as the "Sig Brace," or a pistol brace, wrote a letter to ATF Jan. 5 2017 asking for clarification of the ATFs 2015 open letter regarding the redesign of "Stabilizing Braces" into full blown stocks when used against the shoulder. Not so good if you had it on a pistol like everyone and their mother, because now you're violating the NFA everytime the brace so much as makes contact with your shoulder. As is typical with anything ATF said or did during the Obama administration, much of the industry panicked, while others simply shrugged their shoulders at the stupidity and lack of logic the ATF continually possesses in these matters.
Dated March 21 2017, the ATF responded, reversing the 2015 open letter's opinion, stating that "...the January 2015 Open Letter implied or has been construed to hold that incidental, sporadic, or situational “use” of an arm-brace (in its original approved configuration) equipped firearm from a firing position at or near the shoulder was sufficient to constitute a “redesign”, such interpretations are incorrect and not consistent with ATF’s interpretation of the statute or the manner in which is has been historically enforced."
Long story short, I guess you can just go ahead and shoulder that SB Tactical Brace pretty safely (IANAL). I don't know about the GHW Tailhook (which looks awesome), or the Shockwave Blade, but I sure as hell wouldn't go writing 20,000 letters to the ATF asking for their permission. Let's just move along, and not write any further letters asking for clarification.
Here is a link to the letter itself: http://docdro.id/F3zaZ5b