hardback vs hardbacked

hardback is a noun but hardbacked is not a noun.

Word	Noun	Adjective	Verb	Adverb
hardback	Yes	Yes	No	No
hardbacked	No	Yes	No	No

As adjectives, hardback and hardbacked are synonyms defined as:

Other synonyms of hardback include hardbound, hardcover.

hardback (noun)	hardbacked (noun)
a book with cardboard or cloth or leather covers

hardback (adjective)	hardbacked (adjective)
having a hard back or cover	having a hard back or cover