It's one of these:https://www.norvilmotorcycle.co.uk/techtalk05.htmBought one to go on a pair of Manx Roadholders on a café racer big Oilfield I had until my back couldn't cope any more, and have similar (older and without internal plate) on my 650SS. They are fairly good. (As long as you don't want to stop if rolling backwards on a steep slope - Why would you? )