The ingot, which has been confiscated by Argentina's tax authorities, weighed 25 kilogrammes and was stamped with the inscription "Central Bank of Paraguay 1824."
It is estimated to be worth USD 2.27 million and may also be historically significant, said the Federal Public Revenue Administration (AFIP) in Buenos Aires.
The two Paraguayan men, who reside in Argentina, were transporting the ingot in a truck across the bridge between the Argentine city of Posadas and Encarnacion in Paraguay when customs officers stopped them for a routine search yesterday.
One of the men told authorities they had bought the ingot from a group of indigenous people for 250 million Paraguayan guarani, or about USD 50,000, AFP said.
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Officials also seized about a kilogramme of silver.
AFIP said it had informed the Paraguayan central bank and the embassy in Buenos Aires.
Paraguay, which gained independence from Spain in 1811, fought Argentina, Brazil and Uruguay from 1864 to 1870 in the conflict known as the War of the Triple Alliance.
About 85 per cent of Paraguay's men died in the war. Today, the landlocked country is one of the poorest in Latin America.