In addition to Paviot and Claeys, several authors throughout the centuries have referred to Willem as Guilherme Casmaca or Cosmacra (a possible linguistical corruption of Kersemakere), with the earliest attestation, besides the will of his wife, being a description of his life and family by Gaspar Frutuoso written in the years 1586-1590, in chapter 36 of the manuscript Saudades da Terra. Likewise, in a contemporary account to Willem's lifetime, Valentim Fernandes refers to him as "'Guylelmo Hersmacher'" in his latin manuscript "'Descripcam de Cepta por sua Costa de Mauritania e Ethiopia'" published in 1506.