I quite agree Ross, the best proof would have to be in the provenance, (history).
If the gun was made with tools available to the era, (not unthinkable, where a million dollar price tag is involved,) finding *some* mention of it somewhere would be a small comfort.
If it was fabricated from scratch, and done with period correct tooling and finishes, there would be no Carbon 14 type dating that would apply, as it would not be organic material.
X rays and magnafluxing would only show a frame modification, not a completely new build.
Without provenance, it falls into the category of "extremely suspicious".
It is no higher standard that I hold this alleged .45 carbine to, than the
world would hold any as-yet-unheard-of other historic, artistic, or
genealogical treasure.
It is quite equivalent to some art dealer claiming to have found an
undiscovered DaVinci miniature baseball card sized painting of the Mona
Lisa. Most experts, rightfully, would say "Hogwash, prove it!"
It is equivalent to the old senile woman who claimed to be the lost Russian
princess Anastasia, who was debunked by genetic tests.
It is also equivalent to the "Hitler Diaries", that were greeted with
skepticism, auctioned for a ridiculous sum at Sotheby's, and also proven to be faked.
Without a solid line of provenance, we all know how age and metallurgy can
be faked. And the fact that not ONCE was this gun ever mentioned in any
records is a bit far fetched also.
This logic is not "guilty until proven innocent", this is good healthy
skepticism of outlandish and unlikely claims.
To act otherwise in the world of collectibles would be gullible.
The burden of proof lies with the seller.