Is there a set checklist to prove the life of an historical firearm?

I am trying to help with getting paperwork on a piece of history for a family friend.

Would the story of a surviving relative be enough to prove where it came from?

It might be possible to contact the manufacturer of the gun, but I don't think the engraving is factory,
and at best I would get a born on date.

Thanks.

"Would the story of a surviving relative be enough to prove where it came from?"

Not to me or any but the most gullible buyer. You Mention engraving, most engravers leave their signature or initials in the work and even if they didn't fine engraving can still dramatically increase the value of a gun.