Looking at these cores on BBR.com I can't really see anything obvious that makes them asymmetrical.  What is missing in the drawings that would show us that they were, in fact, asymmetrical?
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It's kind of hard to read the lanes if you don't know their language...

The Phantom's were twin-pin balls...they would only have been symetrical if sliced through one of the pins.

I don't recall people talking about MB back then...the twin pin was sort of a radical thing in 1990 or 1991.  Exoctic drills were still relatively uncommon when the Phantom's came to the market.  

I remember some drillers not being real certain how best to lay them out and I was fortunate enough to be bowling in a house that was home to a Brunswick staffer that year who was also doing very well that season

The PSA locator and mb markings go hand and hand.  I think what made these cores assymetrical is that there were raised rings on one side of the core(similar to what roto-grip did in their "Fuel" series balls).
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-DP3