I have been offered what I've been told is a "falling-block single shot rifle by George Gibbs in .500 cordite" the first thing that came to mind was the pretty picture of a Farquharson, something I've always wanted to own

Never heard of any Farquharson's in .500 NE, much less seen one. Can anyone shed some light on this? Could it be a BP rifle in .500 or .577/.500?