I am not sure, but think the Mark 5 ammo is what I shot when I had an inexpensive Stevens SxS when I must have been about 15. All I knew about shotguns then was you pull the trigger and it goes boom. Come to think of it, that is all I know now.
The pre-grey-bread crowd may not appreciate what a quantum leap it was to go from paper hulls with card and fiber wads to plastic hulls with one-piece wads. Back in the day, the new, more consistent, loads caused competitive scores to skyrocket and gave cleaner kills in the field. And, when shooting in damp conditions, jammed guns (because of swollen paper hulls) were history.