The map, marred by an inkwell accident in 1869, has a note by the Azores archipelago, presumably written by Vallseca, stating: Aquestes isles foram trobades p diego de ???
[1][2] It has since been read by other investigators as Diego de Senill [3] ('the Old' - a hopeful reference in the direction of Gonçalo Velho, who officially discovered the Azores in 1431).
It is often assumed (albeit without corroboration) that Diogo de Silves was a captain in the service of the Portuguese prince Henry the Navigator.
If so, he may have been sent out in 1427 as just one of Henry's several expeditions in the 1420s down the West African coast in an attempt to double Cape Bojador, or that he may have been going on a routine trip to Madeira, and it has even been speculated he might have been part of a failed Portuguese attack or slave raid on the Canary Islands.
How he ended up in the Azores is uncertain - he may have been blown off course, or may have been gathering intelligence about oceanic winds and currents, perhaps experimenting with one of the earliest volta do mar routes for Henry.