Since your gun did not leave the factory configured as an assault weapon, you would have to have some proof that you had assembled it as such before 9/14/94.  Otherwise, the gun is NOT a grandfathered assault weapon, and is "post-ban" regardless of when it was made.

-Troy

Yes, but because YOU were the owner on 9/13/94 (and can prove it), you are in a legal position to testify to the gun's configuration on that date.

Someone who bought a pre-94-manufacture Mini 14 in, say, 1996, would need the receipts or some other written proof.  This is because the current owner didn't legally possess the rifle on 9/13/94, and so couldn't legally testify to its configuration and legality on that date.

I recommend saving your receipt, and perhaps even making up an affidavit stating that you were the legal owner of the rifle on 9/13/94, and that it was in AW configuration (name the individual features) on that date.  That way, if anything happens to you, or you want to sell the rifle, its pre-ban status won't be at risk.

-Troy

BATF is still bound by "innocent until proven guilty."  The problem is that you mis-understand what it is that they have to prove you guilty of.

The prosecution needs only to prove you were in possession of a gun configured as an "assault weapon."  That is their burden of proof.  If they can prove this, they have a case.

As part of your defense, your lawyer can assert that your gun was grandfathered under the listed exceptions in the law.  However, it is YOUR burden to prove your defense.

-Troy