The inscription on the stamp gives CORSARIO ARTIGUISTA what means a “corsair of Artiguista”, not a special vessel is depict on the stamp.
Corsair means a licensed sea raider and a synonym for “privateer” a ship marauding with governmental approval. Artiguista means a vessel belonging to Artigas.
Jose Gervasio Artigas (1764-1850 was the first leader in the movement toward independence of Uruguay, and is a national hero of that country.
When he not could afford a navy fleet the only means for harassing his opponents Spain and Portugal was to form a fleet of corsair vessels to decimate the opposing commerce.
The first privateers who hoisted the flag of the free protectorate Banda Oriental were the SABEIRO and VALIENTE.
When in 1820 the Portuguese captured the country and annexed it to Brazil, Artigas spent his remaining years in exile in Paraguay.
Source: Some web-sites. Mario Rosner. The Wordsworth Dictionary of Pirates by Jan Rogozinski.