It should be a Winchester 12 gauge shell that was made between 1894 and 1900 or possibly in the early to mid 1920s. Between 1901 and 1920, the headstamp was "1901" instead of "Winchester". In 1921, they went back to "Winchester" and sometime in the '20s, they changed from "No." to "GA".
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae