I keep seeing some basses with what is apparently an optional rest of some kind that's screwed into place so that's it's on the pick guard to the right of the G string (on a right handed bass).
Neither of my basses have it and a search on Google for anatomy of a bass only showed basses that didn't have it either. If it were a rest I'd have expected it to be for the thumb, but this clearly isn't the case since it's on the wrong side.
What is it and what's it for?
There is a good look at a bass with it here:
http://www.talkbass.com/forum/f8/pawn-shop-find-753854/#post10636912
Neither of my basses have it and a search on Google for anatomy of a bass only showed basses that didn't have it either. If it were a rest I'd have expected it to be for the thumb, but this clearly isn't the case since it's on the wrong side.
What is it and what's it for?
There is a good look at a bass with it here:
http://www.talkbass.com/forum/f8/pawn-shop-find-753854/#post10636912