letters of marque vs letter of mark and reprisal

letters of marque and letter of mark and reprisal both are nouns.

Word	Noun	Adjective	Verb	Adverb
letters of marque	Yes	No	No	No
letter of mark and reprisal	Yes	No	No	No

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

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

Other synonyms of letters of marque include letter of marque.

letters of marque (noun)	letter of mark and reprisal (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