carlo: If it were me, I'd verify that the serial number is not post-ban, and get a signed statement from someone who actually owned the rifle before 9/13/94 that it is grandfathered. That means it was either assembled into a pre-ban configuration, or was set aside with all parts necessary to make it into a pre-ban configuration on or before that date. A letter saying it's "pre-ban" is meaningless as proof unless it's from someone who had personal knowledge of the gun's configuration.
So, if the serial number doesn't make it a provable post-ban, and the letter you're talking about is from someone who has personal knowledge of the rifle's grandfathered status, then I would consider that sufficient proof for myself to make that purchase.
There is no federal law stating that a private owner must have proof of a weapon's pre-ban grandfathered status, but that proof is your only defense if you were to be charged with possession of an illegal AW.