If you try the search function, there are several threads talking about this, there is also a video showing how they are installed.
they can easily be put in a existing drilling, basically they drill the thumbhole a little larger and install it.
heres a 4 page manual that shows how the VISE Interchangeable Thumb (IT) is installed:
http://www.viseinserts.com/media/it_...ion_manual.pdf
Cost around $25 for the inner piece and $15 for the outer, but it will vary from shop to shop.
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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