The reason the bonanza balls are legal, but the bonanzaII balls aren't is because the bonanza balls stick to the original core/cover combination, while the bonanzaII's may have different cores inside than normal.
Having a different core inside actually constitutes a seperate and completely different ball than the original, and one that may not have been approved by the U.S.B.C., that is why the bonanzaII's aren't legal.
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"The difference between stupidity and genius is that genius has its limits."
Albert Einstein