It's a halflink for 1/8" chain, which makes it a useful object for getting your rear wheel optimally far forward and so on.

But also it's a screw-fastened masterlink, meaning that you can demount the chain in seconds using a screwdriver and a set of needle nose pliers.

The link was developed for track use, so we recommend regular checks and the use of Loctite if you're planning to take it on the road.

Fixing screw is silver on some stock!