I just got this and installed it as a "tug bar" instead of a thumb-rest (ala 60's Jazz Bass). The original placement for this piece was below the G string so that you had a place for your fingers while you played with your thumb, which is how bassists who were used to playing double-bass used to do it. It was moved above the E string as a thumb rest in the 70's, and discontinued in the 80's.

The template that comes in the package only covers the thumb-rest placement (above the E string) for both the Jazz and Precision basses, so if you want to get the cool 60's vibe (50's for the Precision) and place it below the G string, you have to eyeball it yourself. Fortunately, there are enough detailed pictures on the Fender site of re-issue bases that have the "tug bar" that it isn't too hard. I did it, and now my Mexi Fretless Jazz Bass looks like a 60's reissue.

I put this on my Fender jazz bass, comes with a diagram that you cutout and lay on your pickguard so you can position it correctly if you want to put it where they sugest. Just line up the marks drill a couple smaller holes than the screws and Wha......llll you got yourself a place to rest your thumb.

Thumb rest. That's what it does.