I believe that NoSecondBest is right.
I found a post that describes stepped base sabots as being made by MMP:
However Knight also made their own very thick high pressure sabots that they eventually also had MMP produce exclusively for them. There's some specifications about them listed in a chart on page two of the following thread. It doesn't mention whether they also have a stepped base or not.
It's possible that some of the older MMP sabots were thicker than some of their current production sabots.
And according to this chart, some MMP stepped sabots had machine numbers stamped inside of their base cup.