brass knucks vs brass knuckles

brass knucks and brass knuckles both are nouns.

Word	Noun	Adjective	Verb	Adverb
brass knucks	Yes	No	No	No
brass knuckles	Yes	No	No	No

As nouns, brass knucks and brass knuckles are synonyms defined as:

brass knucks and brass knuckles: (used in the plural) a small metal weapon; worn over the knuckles on the back of the hand

Other synonyms of brass knucks include knuckle duster, knuckles, knucks.

brass knucks (noun)	brass knuckles (noun)
(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand	(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand