plural form vs brass knuckles

plural form and brass knuckles both are nouns.

Word	Noun	Adjective	Verb	Adverb
plural form	Yes	No	No	No
brass knuckles	Yes	No	No	No

plural form (noun)	brass knuckles (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