letters of marque vs letter of marque

letters of marque and letter of marque both are nouns.

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

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

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

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

letters of marque (noun)	letter 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