Our Mercury started life as a Nigerian Police bike. We suspect Mercurys, being sold alongside the early Commandos- our's was despatched in 1970-, simply didn't sell. So they were unloaded to police forces and other gullible entities.
That said, the legendary John Hudson, Service Manager of Norton, owned and rode one when I first met him at "Parkroad Motorcycles" in Southampton in1979 where he worked after Norton closed down.
The Mercury is a detuned version of the 650 Dommi with a single carb. A very nice, mild bike and most certainly not a roadburner.
Fun to ride at normal speeds on B roads.