1975
I'd like to start off with the lunchbox from my personal collection that I took to school each day back in 1975:
The helmets from the NFC teams are on one side (including the one year design the Giants used in 1975), and the AFC teams are on the other:
The sides have red and blue stripes going around the lunchbox:
And just to make it all the more authentic, its still got one of the Wacky Packages I stuck on it almost 35 years ago:
Wacky Packages were the first thing I ever collected, so this lunch box is a great example of 2 things I really liked back when I was a kid. As I recall, I think I got at least 2 years of use out of this lunchbox as I did not get the 1976 version. Unfortunately I don't have the thermos any more, but I did find someone selling this lunchbox with the thermos on ebay, so here is a look at what the thermos looked like:
1964
Going back to the very first NFL lunchbox, the 1964 lunchbox is fantastic. From the Bears vs. Packers action on the front

to the Giants vs. Browns action on the other side

to the team logos and uniforms of each team around the sides:




this is truely an awesome lunchbox. To make it even better, the thermos is great as well, with a series of referee signals:



The lunchbox has a 1964 NFL Properties copyright:

I'm not sure if this lunchbox was only available in 1964, or whether it was a design that was used for a few years afterwards as well. Since there don't seem to be any different NFL designs until we get to the 1970's, I'm guessing this design may have been used for a couple of years.
Early 1970's
The next NFL lunchbox design was produced after the AFL-NFL merger and had the AFC teams on one side
Given the Chargers and Eagles helmets are white, I'm guessing that this lunchbox is from either 1972 or 1973 (since the Chargers helmet changed to blue and the Eagles changed to green in 1974).
The top and the sides of the lunchbox have yardlines and an end zone
while the bottom has the team names:
1976
One year after the 1975 lunchbox I profiled above, a new lunchbox design was created to include the 2 expansion teams, again with the AFC on one side:
and the NFC on the other:
The sides of the lunchbox have a generic AFC and NFC player:
1978
The final NFL lunchbox design put the team's helmets on the sides
and had a painting of various NFL players (including Tony Dorsett, Franco Harris, Gene Upshaw, and Pat Haden) on the front
while on the back was a very uninteresting generic helmet with the NFL logo on it:
Early 1980's
It appears that the 1978 design was used for a few years as there is also a version with the Bengals helmet with stripes which would date this lunchbox from 1981 at the earliest (when the Bengals made the change to their helmet):
It appears this was to be the final design for NFL lunch boxes, at least in terms of those made out of metal.
It looks like NFL lunchboxes today are for a specific team instead of designed to promote the entire league:

and the days of metal lunch boxes are now long gone.