From the letter:
Attorney General plans retroactively to exempt such firearms from the collection of NFA taxes if they were made or acquired, prior to the publication of this notice, in good faith.
You have to have already owned the stock before publication of the letter, and purchased it in good faith to qualify for the free stamp.
What's to keep people from buying every brace in existence between now and the opening of the registry? What's to say I don't swap my brace between multiple firearms? What's to say the AG even has the power to declare a tax amnesty?
Lots of ways for this to go sideways unfortunately.