The notice is published in the federal register here, along with instructions for sending comments.
The GCA and NFA generally regulate “firearms” and not individual components and, as such, ATF does not classify unregulated components or accessories alone. However, components or accessories can affect the overall classification of a firearm because: (1) How a component or accessory is actually used may be relevant in assessing the manufacturer's or maker's intent with respect to the design of a firearm; or (2) the design of a component or accessory may result in a firearm falling within a particular statutory definition. Stabilizing braces are one such component or accessory that ATF has encountered.