Bud Orr is old and probably forgot about the others made. maybe the serial numbers were picked by the original owners, so you have 27, that one is 00. personalized serials numbers are not uncommon.
if you want to prove the seller wrong about something that's one thing. but to continue with this pandering, is well stupid.
if people want to believe that its a 1 of 1 so be it. then they will pay for it. its Dave Bains' collection and that in itself is a bit of history. it might be a professional history, but history none the less.
so either get in, or drop it. only until recently where companies kept better records of the guns produced, can you have proven LE and SE with providence.
but at the end of day, each and every gun you own is a 1 of 1. as that's your own gun. put different grips on it and no one else has your gun.