letter of mark and reprisal vs letters of marque

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

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

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

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

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

letter of mark and reprisal (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