Yup, did a quick search for a more recent ATF letter before posting my reply and couldn't find any. It doesn't mean one doesn't exist, just that I couldn't find it after a 15 minute search or so. It's why I mentioned those interested in mounting an angle brace should do their own research. I did find plenty of recent forums 2018-2019 pointing to the 2010-2011 letter as being the latest, but that's hardly an official declaration. Interesting, I found a recent article about the ATF bring charges against an Ohio man that owned a AR pistol with brace, angle grip and Flash Suppressor (supposedly all legal under their own guidelines). They claimed the parts added, made it a SBR, but he was found not guilty of the charges by a jury. So even if I found a more recent letter indicating it was OK, it's apparent the ATF yahoos, could still try to prosecute someone that was within the law, putting ones hope on a Jury or Judge.
ATF Prosecutes Ohio Man for AR Pistol with Brace
The Bureau of Alcohol, Tobacco, Firearms and Explosives has defied its own definitions, saying that adding a pistol brace to a pistol made it a short-barreled rifle.www.range365.com
Yep the Ohio incident is well known & quite the cluster f... in various ways IMO.