"a certain Gentleman Unknown"
Portrait of a Gentleman
Public DomainPortrait of a Gentleman - Credit: El Greco

'Portrait of a Gentleman Unknown' is a common description of a painting in which the sitter has not been identified.

The examples shown here, by Robert Peake the Elder and El Greco, might easily fit the bill for the portrait sitter that the narrator of Rebecca has in mind: 'in black, with lace at his throat' and 'eyes that followed one'.

 

Portrait of an Unknown Gentleman
Public DomainPortrait of an Unknown Gentleman - Credit: Robert Peake the Elder