What is clear the FDA apparently does not know the law, know its own precedents, or keep good records when it comes to NAC.
Let's take a look at that 1963 drug. The Council for Responsible Nutrition obtained the record on that drug via a Freedom of Information Act request. The date was handwritten. No one knows why is was handwritten, when it was written, or who wrote it. Therefore its authenticity is highly suspect. Even more so since the FDA would raise the same concerns if a supplement company submitted such a document claiming that an ingredient was available as a dietary supplement prior to a drug. Further casting doubt on it, is the fact that the only other record CRN was able to find about NAC prior to the July 202 letters the FDA sent out stated they "first approved N-Acetylcysteine (NAC) as a new drug in 1985." That's 22 years later. Further confounding that, the FDA's Approved Drug Products with Therapeutic Evaluations, aka "the Orange Book", does not contain any drug with NAC listed as the active ingredient with an approval date in 1985.
Getting back to that 1963 drug, assuming the document mentioned above is authentic, the drug was inhaled. The FDA treats different delivery methods of the same drug as different "articles." The drug referred to in that 1963 document was inhaled, thus could not apply to any supplement since by legal definition supplements have to be "intended for ingestion". There is a drug listed with a 1982 approval date for "inhalation, oral" but was discontinued in August 1994. The question the FDA needs to answer would be if this drug could be considered the same article given the dual route of exposure. There is a clause that would allow for an exemption but the FDA would have to show that substantial clinical investigations are actually being conducted and are made public before NAC hit the market as a supplement.
The law being cited, the Dietary Supplement Health & Education Act, does not contain an explicit retroactive clause. As such, it does not apply to anything on the market prior to October 25th, 1994 when the DSHEA went into effect. US Courts have upheld that laws cannot be applied retroactively without an explicit intent to do so from Congress/legislatures/etc. I know for a fact that Nutraceutical Corp has had their NAC product for sale under their Solaray label prior and they intend to keep selling it.
The big question here too is "Why now?" There have been hundreds of NAC products put on the market, and the FDA has received New Dietary Ingredient Notifications for them. They had multiple chances to object and yet they let those products go to market anyways.
On June 1st, CRN submitted a Citizen Petition requesting the FDA reverse course.