headpiece vs headstall

headpiece and headstall both are nouns.

Word	Noun	Adjective	Verb	Adverb
headpiece	Yes	No	No	No
headstall	Yes	No	No	No

As nouns, headpiece and headstall are synonyms defined as:

headpiece and headstall: the band that is the part of a bridle that fits around a horse's head

headpiece (noun)	headstall (noun)
a protective helmet for the head	the band that is the part of a bridle that fits around a horse's head
the band that is the part of a bridle that fits around a horse's head