Although according to professor C. Magbaily Fyle this could have possibly been a misinterpretation of historians; there has been evidence of Serra Lyoa being mentioned prior to 1462, the year when de Sintra's expedition reached the coast of Sierra Leone.
The British, prior to the area being colonised, officially adopted the name Sierra Leone in 1787.
[3] De Sintra continued his journey from Sierra Leone to contemporary Liberia, and passed Cape Mount, Cape Corso (aka Cape Mesurado) and stopped at some forested hills, which he called Bosco de Santa Maria- probably contemporary 'Bassa Hills', where he turned since his interpreters could not understand the local language.
[5][circular reference] Pero de Sintra's voyage is the first and only one commanded by the new king Alfons V - who, however, in 1468 negotiated a contract with the private merchant Fernao Gomes to fit out caravels for further exploration of the West African coast (1468-1474).
[7] Gasparrini Leporace, Tullia Le Navigazioni Atlantiche del Veneziano Alvise da Mosto, 1966, Roma: Istituto Poligrafico dello Stato; João de Barros, Decadas da Asia, Nova edição, Lisbon, 1778 Decada I, livro II, cap.II, 142 (in archive.org)