Yeah, they are rare but you really have to be able to prove it with something...maybe the legit "ARRCO" sticker, or a gut shot like stated above...there are some 'tells' with Jose's work that Cameron, Fortin or Friedman might be able to spy and prove/disprove it's authenticity.
But he says "Dumble category"...I don't know, all Dumbles sound pretty incredible while there are some Jose dogs out there.
I will say the last one I saw, on a Nashville craigslist about 10 years ago went for 5K and gone in a day. But he had some provenance with it plus the real Arrco sticker on the chassis.