[shrubble]

The usual claim, which is difficult to prove without physical access to the bars, is that some bars were made by melting and casting the confiscated gold coins that FDR gathered in the 1930s.

Such bars would not be to the 99.9 percent gold standard set by the London Bullion Market Association. They would instead be at about 90% purity, since American gold coins had 10% copper added, which makes the gold harder and more wear-resistant.

See https://www.bundesbank.de/en/press/press-releases/bundesbank... however, no explanation is given, only that the bars were melted, purified and then recast.