Where does the yield come from?

“Investigations after the war revealed that the Reichsbank had used large quantities of gold stolen from central banks in the occupied territories to make wartime payments to a number of institutions including the Swiss National Bank and the BIS. This gold had been remelted at the Prussian mint to conceal its origin. In this manner, during the war, the BIS received 3.7 tonnes of gold from the Reichsbank which, it emerged from German records captured after the war, had originally been looted from the central banks of Belgium and the Netherlands. The BIS cooperated fully with the postwar investigations led by the Allied Tripartite Commission for the Restitution of Monetary Gold and returned 3.7 tonnes of gold to the Commission in 1948.”