Different rules made it a completely different game. Defensive backs could hit the receivers all the way down the field until the pass was on its way. No 5-yard illegal contact penalty. Offensive linemen couldn't use their hands. The standard way if blocking today was holding back then. Heck, the officials today don't even call holding under today's rules most of the time. (Unless the team I'm rooting for scores a touchdown. 😲) The quarterbacks weren't protected from hits the way they are today. Even throwing the ball away took skill, as a receiver had to be in the area to avoid intentional grounding. No "outside of the tackle box." For that matter, even encroachment was called differently. If a defensive player jumped and got back before the snap, he was OK, as long as he didn't contact anybody. If he caused an offensive player to move, the offensive player was called for a false start. Those old rules favored the defense and today's rules are safer in general. We really can't compare statistics between eras when the rules and conditions were so completely different.