The refinery seems to keep a registry of what they have produced (in the case of the Swiss refinery in the article). As a a result, the forged bars ended up hav

The refinery seems to keep a registry of what they have produced (in the case of the Swiss refinery in the article). As a a result, the forged bars ended up having a duplicate serial number which is how they found out they were getting high-quality gold in with forged refinery stamps.