Silencers and short barrels are not specifically listed. Just sayin.
I would believe this falls under the phrase "NFA firearm". And it does specifically mention applications to make a firearm, which is a Form 1. While a complete firearm would be transferred on a Form 4, if anyone wants to make their own SBR it is filed on a Form 1.
I would say the descriptions in this proposal are vague for a reason, and it is not friendly to the legal gunowner.