More great info. To be fair the only things that I have made up my mind about are:
1. There was no violation of the law in my case.
2. It should not be common practice to bug me simply because I am in possession of a NFA weapon, regardless of what usually happens.
I certainly didn't ask the question having already possessed the answer. I think the question has been resolved as I thought it would, and the vast majority of the thread ended up delving into areas that are somewhat grayer than one might think, evidenced by the 100+ posts and several thousand inquiring minds who have been following along.
I will happily carry my forms with me from now on, and present them when asked, as I do believe that Fiddle and others have made great points that would support an arrest in this case, though I still don't believe that it would perhaps be the correct course of action for a LEO to take. Just because something might be legally permissible, doesn't make it the right thing to do (as in the case of pulling over everyone who drives just because you can.)
In any case, we are all absent minded at times, and I think it is not appropriate that forgetting the paperwork for a legally owned firearm should lead to an arrest on a felony charge, regardless of whether your affirmative defense would get you off the hook eventually. I think everyone would agree that what would make this situation a non issue would be an accessible database for law enforcement that would allow them to check the status of a weapon in the same manner as they can check for your driver's license.
However, now we get back to the issue of what the federal government really meant to achieve with the registration of the NFA firearm through the issuance of a tax stamp. End run around the second amendment maybe? I think this is such a unique situation that no analogy really fits exactly, be it the car or the marijuana case, so yes I suppose we will have to wait and see what happens when such an arrest goes down, though I find it hard to believe it hasn't already happened.
In any case, I have found this to be a great exercise in working out the details of the situation through the open exchange of ideas between people who have been on all sides of the situation, hopefully to the benefit of each one of us.