As I understand it the reason for the change from OZ of shot to pressure was that the amount of shot does not automatically guarantee the pressure. 1 oz of shot could be loaded to an unacceptable pressure level while a heavier shot load could be loaded within an acceptable pressure level. Marking the maximum service pressure was thus a more significant parameter than marking the shot load.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra