When it comes to spending big bucks on any expensive antique provenance is everything. A well educated guy with plenty of money can obviously take a risk, but why take a risk?. We know that the NBTHK and NTHK papers are only the opinions of educated folk, but they are widely accepted as provenance. The sword you where talking about without papers, maybe the dealer guaranteed papers for the client.
I watched a documentary a few weeks back about a painting that had been declassified as being by Gainsborough. An expert in Gainsborough thought the decision was wrong and tried throughout the show to disprove the decision and get it reinstated. The scenery was typical Gainsborough, there was even two characters in the picture that where a match for another Gainsborough picture, apparently artists do this quite often. The expert was sure, but the test on the blue used proved it was cobalt blue, a later type of paint. Anyway, gone off topic but watching this show made me realize the importance of provenance.
Ps, turns out the picture was not a copy, but a Gainsborough drawing that someone over painted in a gainsborough style some years later. Even so, now its back in a Gainsborough museum with a nice new value$$$$$, hope you get my point.
Apologies for wondering off topic!