The ATF does NOT buy into the notion of exchanging one EVIL feature for another!in regards of the AR and its design...
The addition of a thumbhole stock to a Post Ban rifle will not allow you to "exchange"
one feature for another, you cannot legaly add a "flash hider"to a post ban only ATF approved muzzle brakes. if you had a pre ban barrel assembly with [bayo lug,threads]you would have to remove the lug and then you could attach a threaded muzzle brake by "blind pinning and silver solder" which is drilling a hole through the brake and partway in the barrel then installing a roll pin followed by silver solder[WHEW!!]
Also for future reference, A "so called" Pre Ban lower/receiver" cannot be configured into a pre ban featured rifle! only post ban.
The Receiver must have been assembled into a rifle prior to Sept 94 by the manufacturer to legally qualify as a Pre Ban..
And yes I am a refugee from AR15.com