Burke's Genealogical & Heraldic Guide to the Peerage, @ Sir Hugh de Plessetis, no parents
The cited information was published by Burke's Peerage, London, 1949 (Ref: Sir Hugh de Plessetis, no parents) The author/originator was Burke's Peerage.
Source Notes
TITL Burke's Genealogical & Heraldic Guide to the Peerage, Baronetage & KnightageABBR Burke's Peerage