I would assume real gold bars of this size would have some kind of notion of a certificate of provenance and/or authenticity stamped into it or associated with it, e.g. specific symbols or ID numbers that would fall under international forgery laws if duplicated. Because one and 10 troy oz bars come with loads of identification. Of course those can be faked too, but who would be dumb enough to trade in dubious unmarked "gold bars"? The only answers I can come up with sound like James Bond plots.