Uncle Willy förklarade det så här:
Question: How do you decide which game is manufactured under the Bally label or which one under the Williams label?
Answer: This has been one of the industry's most closely guarded secrets. Unfortunately Pat Lawlor let his guard down at the last Pinball Expo, and revealed the precise formula: "First we make a Williams. Then we make a Bally. Then we make a Williams. Then we make a Bally..."
Uncle Willy can't think of any way to explain it better than that!