I have one in mine. It makes doublestop bends easier but kinda makes the bridge a bit stiffer, especially on the down bend.
Very hard to find in local shops. I hunted every single shop in singapore and nobody even knew what it was. Then I emailed Swee Lee and one guy finally replied saying that he had ONE piece. So I went down and got it from him, I think around $15 - $20. The rest of the sales personel there have not even heard of such a thing.