Some interesting observations here and I would like to add my own. I too have noticed that often on new brass cases purchased today there is some mottling effect already showing up, something you would not normally seen unless on very old cases or cartridges. I think this is because the cases we purchase nowadays are usually bulk packaged with cases lumped together touching each other. I usually clean my cases using Brass Brite, a wipe on wipe off liquid which works instantly even on the dullest case and gives a nice polish. If I keep these cases in a plastic partitioned ammo box they stay shiny all over but if I place them together in a bag they do show slight mottling after relatively short period of time just like new bulk buy brass seems to.

I have a 20+ year old packet of new RWS 404 brass with cases partition in light moulded plastic and these have retained their perfect finish even after all this time. Partition new brass is also the reason we never usually saw defective cases as I suspect they were probably hand packaged not like today's brass, machine poured and weighed into a plastic bag or a box. Every lot I have purchased in the last few years has had at a few cases with manufacturing defects making them unusable, and not just dented mouths which can be ironed out in the resizing die. Four dud cases in my latest purchase of 50 25-06 Winchester brass cases.