He would give a statement under oath, most likely before a notary public.
Of course, the letter could still be admitted for its effect on your knowledge and intent (you thought it was legal), just not for the truth of the matter asserted (that it was in fact pre-ban).
After 20ish to 35ish years the letter becomes admissible for the truth of the matter asserted (depends on the state). Since any trial will likely be in Federal Court, FRE 803(16) says 20 years.
If the statement about the pre-ban status is on the face of the invoice or the receipt, it may get in under FRE 803 (14) or (15), if you claim the deal was for an interest in a pre-ban rifle.
You could also see if he keeps a book or file showing that the gun was assembled pre-ban. You could try to get a copy of that file in under the business records exception (that's getting more uncertain, most courts would want an affidavit from him stating that those records were made in the standard course of his business, the feds want such a statement for sure under FRE 803(6)).