mid-seventies vs seventies

mid-seventies and seventies both are nouns.

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

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

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