I found this on the internet -- All-brass shot-shells were available in the U.S. as early as the middle 1870s. Interestingly, the development of all-brass and paper/brass shot-shells (a paper hull with a brass base) occurred nearly simultaneously; however, the brass shells held the lead in the market (especially with duck hunters) until manufacturers began to coat their paper/brass shells with wax or lacquer to prevent them from swelling. Around 1900 the paper/brass shell began to emerge as the market leader, a position they held until the later advent of the plastic/brass shell.
I understand that brass shot-shells were also used in Viet Nam, and they can still be purchased today, but as far as I know, you can buy the brass shell, but you have to load it yourself. How do you know it's Winchester? i couldn't see anything but the 12 on the base. The 12 means it's a 12 gage shotgun shell. Looking at the patina I'd guess that yours dates back to around 1900, + or - several years.