BFLR: I’m waiting for some smart Freepers to add some insight here...
Silk road and other trade routes probably gave info about the Pacific, but the west coast of South America is unexplained if one continues to insist on no pre-Columbian contact by places like China, Japan and India.
A Chinese emperor’s fleet supposedly circumnavigated the globe before Magellan. If so, and due to Italy’s trade with China in the 1300s & 1400s, perhaps the map maker had heard of an ocean beyond the Americas.
Well, if you posit that the Americas are a new continent—and you know that India/China abut an ocean—it’s not a big reach to assume that is an ocean between India/china and the Americas.