Re: I'm curious on the vintage of this shell
All military ammo that I have seen has the primer peened in place. In the case of the one you found, it is peened in 3 places. Modern military contract ammo uses a circular type peen. It is not quite as easy to see. Mil spec ammo also has a sealer around the primer to make it more moisture resistant and usually a sealer on the bullet to case joint as well. Maybe that is why the stuff will shoot just fine after 60 - 70 years. I recently fired multiple boxes of 45 ACP ammo that used steel cases and was made in 1943. I shot just fine.
Bob