From Pedigrees of the County Families of Yorkshire
1:
of Aldwark and Ste[v]eton, in right of his wife, died 1478, buried at Tickhill
The Fitwilliam Missal
2 says he died 22 Sept 1479.
Head of the Fitzwilliams of Aldwark, gaining the estates through his wife Elizabeth, heiress of Clarell.
Footnotes
[1] Pedigrees of the County Families of Yorkshire, Joseph Foster, Volume 2 (1874) page 116 (Reygate of Steveton)
[2] The Illuminated Manuscripts in the LIbrary of the Fitzwilliam Museum, Cambridge, William George Searle, 1876, pages 146-149