The interchangeable thumb allows for greater ease in providing the participants with options for different thumb sizes. Without them, you would pretty much have to drill a ton of different balls, rather than just adjusting sizes on a smaller number.
As for Ebonite having a patent, that's not possible. They may have the exclusive right to use the term "demo days" just like Brunswick has the patent on "Cosmic Bowling", but there is no way that they have any right that keeps other companies from setting up similar presentations. That would be like a baseball coach trying to keep other coaches from setting up clinics.
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I can't bowl 300, but I can bench 345 : )
I provide the muscle for the Fellowship of the Saws