seventies vs mid-seventies

seventies and mid-seventies both are nouns.

Word	Noun	Adjective	Verb	Adverb
seventies	Yes	No	No	No
mid-seventies	Yes	No	No	No

As nouns, seventies and mid-seventies are synonyms defined as:

seventies (noun)	mid-seventies (noun)
the time of life between 70 and 80	the time of life between 70 and 80
the decade from 1970 to 1979