Well, several years ago, you had a 12-man roster - period. But there was an injured reserve to be used for INJURED players. What was truly amazing back in those days was how coincidental some of the injuries became. One player would be taken off IR just as another went down with some mysterious strain or ailment. It was actually quite comical to read the medical reports. Honest franchises were disadvantaged at first. And, as you can imagine, there was quite a rash of injuries after the last preseason games as final rosters were set and 1-2 guys from each team always came down with a strained muscle, bent finger, jambed toe, etc. I actually think there was some kind of competition between franchises to come up with the best sounding injuries (probably had interns leafing through medical and anatomical books). It was such a joke, the league just decided to do away with the IR and go to an active/inactive system.
That matches my recollection as well. I remember Larry Miller not using the injured reserved list until the league basically told him to go ahead and use it even if he didn't have injured players.