letter of marque vs letters of marque

letter of marque and letters of marque both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter of marque	Yes	No	No	No
letters of marque	Yes	No	No	No

As nouns, letter of marque and letters of marque are synonyms defined as:

letter of marque and letters of marque: a license to a private citizen to seize vessels of another nation and their property

Other synonyms of letter of marque include letter of mark and reprisal.

letter of marque (noun)	letters of marque (noun)
a license to a private citizen to seize vessels of another nation and their property	a license to a private citizen to seize vessels of another nation and their property