I found a discussion that took place at the 2007 USBC meeting that is related to this topic. It seems the general concensus is a bowler is either right or left handed classified based on the side of his body he delivers the ball from.
However, it is common for a 2-handed bowler to have an additional 'gripping' hole(s) drilled in the center of the grip, presumably for a finger(s) of the opposite hand. This is an additional hole or holes. The USBC discussion ruled that all holes meant for fingers and thumb must be used by the same hand. Ruling the gripping hole illegal.
From the Discussion: While a change was made several years ago to require gripping holes to be covered during a delivery, the current application would still allow a two-handed player to drill a set of finger holes for each hand on opposite sides of the ball. This allows a player to insert fingers from both hands to assist in maintaining the grip and imparting force on the ball.
Any hole that cannot be reasonably shown to be used with a single hand would be classified as a balance hole.
In addition, they discussed the thumb hole drilled for a bowler who used No Thumb. In this instance, the thumb hole was intended to be used for spare shooting.
The resolution was that each hole intended for a finger or thumb had to be covered in the delivery.
From the Discussion: As in the recent past, while not required to be used, all gripping holes must be covered throughout the approach and delivery. This means that a player not using the thumb hole must lay the ball in the palm of his/her hand such that the palm or thumb covers the thumb hole. If the player rotates the ball in manner that results in the thumb hole being fully exposed, they are in violation of USBC rules and following a warning, be subject to disqualification from the competition.
If the unused thumb hole is not covered in delivery, it is considered an extra balance hole, and illegal.
The actual rules have not been updated to include these conditions, but these rulings from discussion may be used until they are formally adopted.
This will be an interesting discussion, ruling, and test.
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