"Proof of custody" is someone certifying that they are holding your assets for you. In this case, International Depository Services of Delaware.
The assets back tokens on Counterparty that you can trade.
It is not really anything new. GLD works just like this except the tokens are called "shares" and they are traded on NYSE. It is also similar to
E-gold and
Liberty Dollar warehouse receipts, both of which were shut down by the U.S.