countersign vs countersignature

countersign and countersignature both are nouns.

Word	Noun	Adjective	Verb	Adverb
countersign	Yes	No	Yes	No
countersignature	Yes	No	No	No

As nouns, countersign and countersignature are synonyms defined as:

countersign and countersignature: a second confirming signature endorsing a document already signed

countersign (noun)	countersignature (noun)
a second confirming signature endorsing a document already signed	a second confirming signature endorsing a document already signed
a secret word or phrase known only to a restricted group

countersign (verb)	countersignature (verb)
add one's signature to after another's to attest authenticity