Paul, if your fitting or attachment is designed to penetrate the coating or paint that is required by 300.6, then you are correct, if not, 250.12 trumps 300.6.
If you don't agree with this, then the two article sections are in contradiction of each other, and I would choose to violate 300.6 and go with 250.12.
Remember a threaded hole is not required for this attachment, it is simply a preference, a bolt and nut can be used.
Roger
