Bullet Bros Chain Tensioner. In good condition comes with 2 strength springs and the rear mech attachment. Finished in a very light blue/turquoise colour!
It mounts through the rear wheel skewer on the rear drop out in the position you see there pointing backwards off the bike. The spring then extends down to the bottom off the rear mech and attachs to it via the small black machined bracket in the pic. Simple and very effective at keeping tension on the chain of the older bikes.
Not my pic but borrowed off someone else to see how it fits!