plural form vs knuckle duster

plural form and knuckle duster both are nouns.

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

plural form (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