This is one link on a drilled thermostat. Doing a search will provide more if you need it.

http://www.bonnevilleclub.com/forum/...led+thermostat

In this picture


Both of them labeled as #3 are for the tensioners. One for the supercharger belt, the other for the accessories belt. Both are reverse threaded, as you turn the bolt counter-clockwise to remove the tension on the belt. If they were standard thread, you'd loosen the bolt before the belt. The #3 next to the #8 is the one that you're working on.