Jack,
Good post. I'll add in a couple of tidbits that my recent re-reading of Hatcher's Notebook suggests may be relevant. The reason military cases have the neck annealing stain is that it was once not standard practice to anneal a case neck after forming the case. This leads to what Hatcher refers to as "season cracking", or the spontaneous cracking of case necks in unfired cartridges with age. Once the military found neck annealing post-forming would prevent that, they began to specify it be done to all military cases. The stain is present partly so ammunition with annealed necks may be distinguished from those without. But also, Hatcher found that unpolished cases that have the oxide layers intact will resist corrosion better than polished ones when exposed to the elements.
Annealing may not have been done to all civilian ammunition between the wars, however. You can usually tell if you hold the case in the light just right. Despite polishing, an annealed area will be colored a slightly lighter yellow.
Another thing to mention is non-corrosive primers. My assumption had long been that mercuric priming had stopped just after the first world war, with chlorate primers replacing them. Not so. Again referencing Hatcher, it turns out that chlorate priming mixes were in use from the nineteenth century. Because purly mercuric primers contaminated brass with mercury, such cases would crack easily and not be reusable. The chlorate formulations were created because they did not damage the brass, keeping it available for reloading. This was something Americans, in particular, liked, as they were likely to reload. Europeans were apparently less likely to reload back then. However, in the early 20th century it was discovered the potassium chloride left by chlorate primers, in the absence of the heavy residue left by black powder to dilute it, was responsible for bore corrosion in smokeless powder guns. Thus, the non-corrosive primer was developed. This was accomplished by going back to the use of fulminate of mercury. So, if you have smokeless commercial ammo from between the world wars, the brass may not be trustworthy if fired with the original primer, assuming it still works.
Fulminate of mercury was done away with as soon as lead styphnate became available because it ages and breaks down fairly rapidly. Some mercuric primers would only keep working for a year or so, which is why the military favored chlorate primers until the early 50's.