That is an early M1880 WRA loading tool.The earliest versions did not have the Winchester name stamped on them.It would originally have been boxed with a de-capping pin and powder scoop for the correct charge.The first catalog listing was Jan.1 1884,the last in the Jan. 1896 catalog, although they were thought to be available before the 1884 date.There were three WRA reloading tools prior to this of different designs.
Oh,it is a reloading tool not a bullet mold.
Winchester catalogued them as "Tool for Reloading Central Fire Cartridges",by the time it was discontinued "The Lever Tool".