[1] The presumption of innocence of course doesn't come from the Constitution, but arguably it may be a matter of due process.
There's no argument about it. Presumption of innocence was long ago settled. The state bears the burden of proving every single element of every crime beyond a reasonable doubt, and may not ever shift that burden to a defendant.
[2] But the magazines being pre-ban sounds like an affirmative defense. Evidence that the magazines were pre-ban would also be within the province of the defendant. So it would appear that at least the defendant would have the burden of putting forth evidence that the magazines were pre-ban.
This does not sound like an affirmative defense to me, although I would have to see both 1) the law, and 2) how it operates. Laws can purport to make an element of a crime an affirmative defense (here by using semantics to say "we're not forcing you to prove you didn't commit a crime, we're forcing you to show you
did something... namely make a certain purchase before a certain date), but the courts will not usually stand for that. I would have a very serious question about any law that would require you to prove you
didn't do something wrong. This is rank burden shifting, unless there's something serious that I'm missing.
Affirmative defenses are more like "Yeah, I did that, but here's the reason I'm not guilty anyway".... Self-defense can fall into this category (although I belive only one state chooses force D to show that in the case of SD). Yes, I was standing over that dead body, and yes I shot him, but he was trying to kill me."
That is an affirmative defense.
Asymmetry of information might shift the burden in a civil case, but I can't rememeber ever hearing of that in a criminal case. Even if it was an affirmative defense, the state cannot make the defendant carry any burden of proof greater than preponderance of the evidence (more likely than not, or 51% likely). Since the state will rarely, if ever, have even the slightest shred of evidence to rebut that defense (and certainly would have no such evidence in the OP's case) it would be up to the defendant's credibility... or maybe practical considerations of how anti-gun the judge or jury was.
Nothing but respect for you. I hope I didn't come across too strongly, but this stuff gets me fired up!