The first thing you notice in these old games is that there is no shepherding or blocking in marking contests. There is no chopping of arms either. The ball is kicked forward, a pack forms, some try to mark or spoil, others try to get front and centre. Nobody tries to keep players away from the ball.
Then you notice the consistency in decisions, probably because there is only one umpire. If a player didn’t quite hang on to a mark in the goal square but he paid it anyway, then he would pay those marks for the rest of the day.
This next point has nothing to do with umpiring but is all about coaching. A player about to be tackled often tapped the ball towards a team mate, or even to his own advantage. Now they take possession and dive onto the ground, hoping to force a stoppage.