I'm not dead sure, but I believe that you do not have to use all of the finger holes in your ball. The holes just have to be drilled in a position where they CAN be used as a finger hole. You cannot have a hole somewhere on the ball and say that it is for your forefinger or pinkie if it is not possible for the hole to be used when throwing the ball. Bowlers who throw the ball without inserting their thumb can have a thumbhole in the ball, as long as it's position is such that their thumb can be inserted in it for ball throwing purposes.
I believe the rule is no more than twelve holes in the ball--one for each finger and thumb, small vent holes for each, a balance hole, and one of those shallow mill holes usually seen only on pro's balls. As I said, finger and thumb holes have to be positioned so they can be used even if you don't actually use them.
If this is wrong, somebody please post a correction. I don't have the rule book with me here at work.
Shiv
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I think we're all Bozos on this bus