"If Yale can't produce a transfer of ownership from Peru, Yale is in possession of stolen property."
Wow. If I visit Mexico or wherever, and buy any native art or handiwork or anything at all really, and do not have a bill of sale that matches the bill of sale that the sellers have, I am in possession of stolen goods.
Seems like all the seller has to do is lose their copy, and I am now in possession of stolen goods. Great racket if you can get into it.