Brass hulls were still made by Alcan up to a few years ago.
I believe that brass hulls can still be had through Old Western Scrounger.
The WW I-era pellets were not, to the best of my knowledge, plated. The ammo was commercial ammo that was simply repackaged for shipment to the troops.
The same loadings were used during WW II, only by that time, brass shells were really falling out of favor.
That caused BIG problems in the Pacific, where due to the high humidity the paper hulls were often swollen before they got to the troops.
The military put out contracts for brass shells; I believe aluminum and steel were also experimented with but none got into service.
In the 1950s the Air Force adopted the Savage survival rifle. Winchester picked up development of the aluminum shells, and developed a .410 load.
During Vietnam the shotgun again was called into service, only this time plastic hulls were commonly available, so there was no need for brass. The brass hulls that were in Vietnam were very likely left-over stores from WW II and Korea.
I have about 2 dozen different brass hulls in my cartridge collection, including some very interesting British stuff for the 14-gauge Greener shotgun.
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Beware the man with the S&W .357 Mag.
Chances are he knows how to use it.