The die is aging. Look on the right side of the 'L' that is extreme die wear. So the die is not a spring chicken. In 1968 they retired the master hub. Why? Because it was spreading outwards. You can start to see this even back then. Moving ahead in time, you the master die was creating the master die with devices moving towards the edge of the die. In 1968, the dies were as far to the edge of the die that they could go.