The VISE Interchangeable Thumb (IT) is the most common one used.
There are 3 main brands:
VISE Interchangeable Thumb (IT)
http://www.viseinserts.com/it.html
TURBOGRIP Switch Grip
http://www.turbogrips.com/products_switch_grip.php
Grip Loc
http://grip-loc.com/
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