Your gun was made circa 1892.I believe that you will find on closer examination that number 57 is in fact 37. These numbers are usually engraved on the trigger guard as,"List 37",rather than item!These numbers represent one of Greener,s many grading systems. This grading system was in use from 1889-1894.
The action you describe is Greener,s patent hammerless cocking system called"The facile Princeps action", It was designed to compete with the Anson and Deeley patent action.This action resulted in a patent suit in which Westley Richards owners of the A&D patent claimed patent infringement. The courts ruled in favour of Greener.
When new, your gun was listed as Medium quality, but at the lower end of the price range.List price was 20 gns.By comparison their best gun was priced at 60 gns.[1 guinea = 1 pound one shilling, stirling]
Pictures of your gun including Proof marks together with barrel dimensions would help to estimate its approximate value.
By chance I examined a similar gun yesterday! This gun was ruined by the addition and removal of a ventilated rib,restocked with a fence post!! triggers mutilated,way out of proof; value almost nothing!Hope yours is all origional?
