brass knucks vs knuckle duster

brass knucks and knuckle duster both are nouns.

Word	Noun	Adjective	Verb	Adverb
brass knucks	Yes	No	No	No
knuckle duster	Yes	No	No	No

As nouns, brass knucks and knuckle duster are synonyms defined as:

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

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

brass knucks (noun)	knuckle duster (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