Yep.
Legally, the sig brace was approved as installed on the sig rifle it was first issued with.
The AR buffer tubes being installed onto non-ars are also a gray area despite them being "pistol tubes." Legally IMO, you're in a much more defensible position if the weapon comes from the manufacturer with the pistol tube and brace installed, especially if that tube is not critical for the function of the weapon. Why? B/c ATF would have approved the entire weapon in that configuration as a pistol prior to import... So you're clear on not "remaking" a weapon and running afoul of the law. When you're adding the tube and brace on your own, it puts the liability on you, which is actually a pretty crappy position for a manufacturer to put customers in. Even the tube itself for instance -- depending on ATF rulings that alone could put you in violation of the law (and I believe the thorsden kit was approved for AR pistols w/ that tube already in place, not for other weapons systems).
I believe all of this was covered previously in an earlier thread in this EVO forum however...
And yes, whether a bare pistol buffer tube touching your shoulder is a stock is also a legal gray area right now with the latest atf rulings. It truly is an unknown. And for the thorsden -- we also don't whether touching shirt or outwear means shouldering or just touching firmly against the shoulder underneath clothing... Or if the weapon recoils and the tube touches the shoulder -- is that remaking? Or is it just when one begins firing with the tube to one's shoulder etc. And once a weapon is shouldered however defined, legally per the ATF, it is forever an SBR. If you don't have or don't file an SBR form prior to that point in time, anyone who owns or shoots the weapon from that point forward until a legal SBR is in place is breaking the law.
Basically, the latest ATF opinion has created a mess such that anyone trying to align the sights with anything less than full arm extension could run afoul of the law. It's ridiculous and I don't anticipate it standing to judicial scrutiny for the exact reasons in the above paragraph...