The term "rare" is over used by sellers on that auction site, usually when they don't really know what they are selling but this Sloper advertised for £100 is unusual and may even be rare.
At first glance it looks much like any other Sloper but look again and it will be seen to have a mechanical governor and a Zenith carburettor more often seen on Suffolks..
The mechanical governor suggests it as being one of the last incarnations of the BSA / Villiers F12 with electronic ignition and engine number starting A15 although everything that I've read says that they have a Villiers Carb.
I was under the impression that I'd never seen one in the metal but was reminded by a former employee that in the 80s we used to hire our a Pattison spiker that had one fitted!!