This is a lacquered brass model or salesman's sample of a clamshell bucket or grabber from a steam powered crane by the firm of Priestman Bros. Ltd., Hull and London, England, circa 1900.  It is pulley operated and works when the chains are manipulated.  It measures 4 x 4 x 5 inches when closed.  I can honestly say I've never seen something quite like it before.  Priestman Bros. were pioneers in clamshell  fitted cranes, so ubiquitous today.  The British weren't known for salesman samples, but they were the finest engineers, so it's possible this was made by the company especially for an important client, much like a shipbuilders model would be presented to a shipping line.

This comes from the estate of a collector who was a mining engineer based in South Africa.