plural vs knuckle duster

plural and knuckle duster both are nouns.

Word	Noun	Adjective	Verb	Adverb
plural	Yes	Yes	No	No
knuckle duster	Yes	No	No	No

plural (noun)	knuckle duster (noun)
the form of a word that is used to denote more than one	(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand

plural (adjective)	knuckle duster (adjective)
grammatical number category referring to two or more items or units
composed of more than one member, set, or kind