I know that most people don't like this, or want to hear it, or whatever, but this is what the lawyers in the Legal forum have advised:
The burden of proof is on the prosecution, but ONLY to prove that the gun in question is configured to meet the criteria of being an "Assault Weapon" under the law. The "default status" of any "Assault Weapon" is banned.
[b]It is then up to the defense to prove that the gun in question meets the criteria to be exempted from the ban under the grandfathering clause of the law.[/b] Evidence meeting the rules of the court (i.e., no "heresy") must be presented to prove the status of the rifle. The prosecution can then challenge this evidence, and so on.
The above is true due to the way the law is written, and this is how our court system works. [b]Based on this, the safest recommendation is to have some kind of written "proof" if you were not in legal possession of the rifle in AW configuration on 9/13/94.[/b]
Note that I'm not suggesting any course of action for anyone; I'm merely passing on reasoned legal advice from those with more experience in this matter than I.
Also note that there have been, to this point, some, but very few people who have been charged with the creation of an illegal AW as the sole charge. Most of the time, this is an "add-on" charge to another, more serious crime. You must evaluate your own "exposure" to the law.
-Troy