Regarding the update on the California situation I note that they say that might be the situation but not for certain that it is for sure. It is a state statute these criminals are being charged with and make no mistake, they are actual criminals. Not just because they had the firearm.
Being from NY I can attest to some similar ridiculous handgun laws in NY. In NY if you were caught with an unregistered pistol you didn't have a permit for you would be looking at jail. However if that is the case as in the case with the updated post on the CA issue why wouldn't they be being charged with the possession of an illegal handgun? Instead they were charged with the having an SBR. Along with the "open letter" (and it's timing) it would seem, to me at least, it's being used as a precedence case to support the ATF's new push to try and put the genie back in the bottle.
I doubt the ATF ever intended for the wrist brace to become a loophole in the SBR rules but clearly many people are using it as such. And I can't really blame them. It's a silly rule in the first place. Now it seems they are pushing to take action on it and put it in writing. First they did it in the letter for the second company wishing to sell a wrist brace and then they didn't it across the board. None of this makes a wrist brace or a pistol it's attached to illegal. Supposedly putting it to your shoulder is now considered "redesign"ing it as an SBR though. I question how enforceable it would be especially if you don't use public ranges with it but I am certainly not going to be a test case for it either.
My personal advice is, if you want an SBR, pay the $200 and do it legally. If you want a pistol to use as a pistol don't worry about it. If in the case of a TEOTWAWKI scenario you want an SBR take your pistol upper and slap it on your carbine lower then because at that time it just won't matter anyway.