20 Réis - João Prince Regent Countermarked X Réis - Maria I, KM# 215

In 1809, the Portuguese crown — then operating from Rio de Janeiro following the Napoleonic invasion of the Iberian Peninsula — faced an acute shortage of small copper currency in Brazil. Rather than strike entirely new coinage, existing X Réis pieces of Maria I were countermarked to raise their face value to 20 Réis, doubling the denomination at a stroke. It was an expedient solution to a circulation crisis in a colony that had just become, effectively, the seat of empire.

The countermark itself is notoriously inconsistently applied, and genuine pieces vary considerably in strike depth.