I understand all that Sam and this first started in 2010 in the Shotgun forum of AR15.com and that letter has been floating around for 6 years with other just as confusing. What I am says is they may be going to end this. There are other sections of the NFA clearly stating that if a weapon has a smooth bore barrel it is either an AOW or an SBS. So, I don't know what they are going to do. As an 07 I refuse to participate in the anything NFA as I am not going to pay them another $1,000 a year for a Tax Stamp. So once this is cleared up for certain I could build these guns without an SOT. I am not going to being playing Loop hole with the ATF as they always win. As a consumer I would by something that could be banned in a few months. Two companies now are pushing the envelop on this so I will watch, wait and see. In the mean time I am going to order extra receivers and barrels just in case as those sell well anyway.
They can change the NFA at their whim. For example: Ever wonder why rifles are 16" and Shotguns are 18"? Originally both were 18", however after WW II the government wanted to sell of a lot of M1 Carbines to help pay the war debit. That Carbine has a 16" barrel so they simply changed the minimum length on rifles to 16 to so citizens could buy them without having to get a tax stamp. So they can change the NFA to suite their own needs any time it suits them.