at a 'loose inch' of freeplay (up and down) the chain should give you a perfect whine on deceleration. a loose chain won't.

They say B/O on the EBR site, but if you pay for one that have consistently "found one" for me pretty quick. 2 times now. Exactly like when I needed the front brake hardware. "We have all the parts in the back, just haven't gotten around to putting them together as kits".

I'd order one. Whats the worst that can happen? You might end up with 2 but it's a wear part, you'll need it someday and its much easier to have it in the garage already.