The color of the corrosion and the extreme brittleness of the brass looks like ammonia damage to me.
That's what propane tank valves look like that have been re-purposed to hold anhydrous ammonia for meth cooks after they steal it from a farmers ammonia pigs.
Why it is concentrated around the shoulder/neck is a puzzlement, for sure!
You have an enemies back then that had access to Brasso and your ammo?
Ed
