Surly Tuggnut Chain Tensioner
Unlike the MKS tugs which we sell to commuters and trackies, the Tuggnut is designed to be used singly. Although you're welcome to buy a pair, we sell them one-by-one. The disk thing is for QR users (instructions enc.) and those projections off the back are bottleopeners. Really.
"The body of the Tuggnut is investment cast in stainless steel for durability. It features a thumbscrew, for tool-free adjustments, and 2 axle holes for a wide range of fore/aft wheel positions. The Tuggnut comes with an aluminum adapter that reduces the axle hole from 10mm to 6mm, so you can use it with quick-release axles, in addition to nutted axles. We sell it as a single, because you really only need one...on the drive side. But, if you absolutely need one on the non-drive side of your rig, to achieve balance in your life, you are in luck...the bottle opener works either way."