There is no black and white answer to that question, but the point is that your proof has to be both submittable in court (i.e., anything that's considered "heresay" can't be admitted), and it has to be strong enough to create "reasonable doubt" in the minds of the jury.

So, what does all that really mean?  Probably, a letter from the manufacturer that the gun left the factory on or before 9/13/94, and was assembled in "AW configuration," or a letter from the person who *legally* possessed the rifle on 9/13/94 that the gun was in AW configuration.

Nothing else I've heard of (or thought of myself) would pass both of the above tests, IMO.

-Troy