Birzman Chain Wear Indicator - road.cc

The Chain Wear Indicator quickly and accurately checked the degree of chain wear and stretch of single to 12 speed derailleur chains.
The indicator comes with integrated chain hooks that help hold chains in place during assembly or repair.

Here is what road.cc says: 

"Birzman's Chain Wear Indicator is a really simple way of telling whether any chain from singlespeed to 12-speed should be changed, and it doubles up as a chain hook to help hold your chain during assembly or repair.