Gem State Airlines - January, 1979

Gem States Airlines served Idaho and Washington locations in this timetable. It would later merge with Air Pacific.

https://en.wikipedia.org/wiki/Gem_State_Airlines
https://en.wikipedia.org/wiki/Air_Pacific_(United_States)

North Central Airlines - June, 1979

Following a merger with Southern Airways in 1979, North Central became Republic Airlines, which merged into Northwest Airlines in 1986. That lineage would make it part of today's Delta Air Lines.

https://en.wikipedia.org/wiki/North_Central_Airlines

Merrimack Airlines - April, 1979

I was able to find this about Merrimack:

Main office in Bedford, Mass. ...aircraft are eight‐passenger Navaho Chieftains ... no attendants or meal service ... seven round trips a day between Westchester and Boston, one a day between Westchester and Albany ... flights continue or connect to Burlington, Vt., Providence, R.I., Bedford, Mass. ... some weekend flights .. . fares $47 one way to Boston, $42 to Albany.

http://www.nytimes.com/1979/08/19/archives/westchester-weekly-commuter-flights-seek-to-fill-alleghany-role.html

Here's what a Chieftain looks like:

Trans Central Airlines - August, 1980

Here's an article about the demise of Trans Central.

Mid Pacific Air - January, 1982

Mid Pacific tried to compete against Aloha and Hawaiian, but ended its island operations in 1988.

https://en.wikipedia.org/wiki/Mid_Pacific_Air

National Airlines - June, 1978

A great timetable from National Airlines.

https://en.wikipedia.org/wiki/National_Airlines_(1934–1980)

Wheeler Airlines - March 1981

Wheeler has an interesting history. You can read it here.

Continental Airlines - October, 1994

This is an interesting timetable. It focuses only on Continental's international routes.

https://en.wikipedia.org/wiki/Continental_Airlines