I have found a handful of 5.56mm cases with this headstamp:
NPA 99 5.56x45

My research has produced mixed information.  The stuff is either South African, and good - if similar to PMP in having thick, heavy cases - or made by S&B for Olympic Arms (Greece) for the Taiwanese National Police Administration.  In this situation, since Olympic didn’t make the brass (yuck!), then it should only have the issues that come with S&B brass, like tight pockets or sometimes small flash holes.

I think that weighing these cases will tell me if they’re extra heavy, but I’d like to hear from folks about their experiences, if any, with this stuff.  I have plenty of brass, so if it’s junk, I don’t have a problem scrapping it, but “I gots to know,”

Curious to see what you find.

Like you, I have plenty of brass.  But unless it's outright defective, I don't scrap any of it - since I've already invested the effort of picking it up and handling it.  So with odd-ball brass that I only have a handful of, I don't care how great it is, it goes in my fire & leave-it pile.  That gets run in a progressive, and when I do run and gun open field shooting, I run that, and just leave it scattered over the acreage.  I find 1000 of them, and that's different :)

I weighed a sample of the cases and they are right around 95 grains each, so they aren't PMP/South African, and I'm calling them S&B cases with interesting markings.

So then I checked the flash holes, and they're all in-spec (my Lyman Flash Hole Uniformer's pilot goes in without resistance).

The pockets all gauge OK with my  Ballistic Tools small primer pocket gauge (which mics at 0.173, the SAAMI minimum diameter).

They were definitely once fired; their three small stab crimps were intact, as was the green lacquer primer sealant.  Decapping pretty much eliminated the stabs, but I may give them a twist with a reamer to make seating a new primer easier.

Like you, lazyengineer, I just don't like wasting usable brass.  I don't want to put a lot of work into oddball brass that has to be segregated because of thick cases or nonstandard pockets, but it took only a few minutes to rule out both issues.  So these 21 cases are in my "next wet tumbling batch" container for when I get around to that.

From what I've read on a cartridge collector site, these were for the Taiwanese NPA. Why they outsourced these when the country has it's own manufacturer puzzles me.

How much does the empty unprimed cases weigh? I had a large quantity of Portuguese FNM-79 brass that weighed 104 grains empty and unprimed. I reduced my standard load 1.0 full grain and it shot fine. I used bulk Speer 52 grain HP bullets and left the brass lay after one reload. I didn't want it getting mixed with USA brand brass.

Do some range testing and reserve it for blasting ammo. It should last a long time if you reduce your normal charge weights. Better than throwing it out during a component shortage.

If you don't want it, there is surely someone in EE will buy it warts and all.

borderpatrol, they weigh around 95 grains.  But I saw quite a few FNM cases in my last batch of processing, so I'm going to go look for them to see if they are also heavy weights.

I don't consider 95 grains an unusual weight for .223/5.56mm. Most of my brass weighs 93 to 95 grains usually.

Yep, that’s why I decided to keep it.

I looked at an FNM case yesterday, and it weighed 99 grains.  I’ll weigh more of them (after cleaning) to make sure these are also ok.