From what I remember, before the NFL implemented the current schedule rotation, they used a system that was based primarily off of "Strength of Schedule".
I don't know how it all worked, but it was far more complicated than today's system. Eventually the NFL saw how absurd it was that Tampa Bay had never played a game in Buffalo. There were other reasons as well. At the time they came up with the new scheduling system, the Packers had not played the Redskins in Washington in 19 years, and Dan Marino & John Elway having played for so many years and in the same conference only played against each other about 5-6 times in the regular season.
The original system was bizarre. I remember there was one season where the Giants & Lions played each other twice.