Thanks!
Unfortunately, I don't have many Optimax codes to compare with, so there's nothing I can say about it (you're probably not very surprised to hear that matrix codes aren't very well documented on the Internet; in the Nirvana community, where I "come from", they are well documented, but Optimax never pressed Nirvana as far as I remember - at least not much). And as the two variations are pressed by different plants, I cannot date them relative to each other.
All I know is that the WEA disc is pressed sometime from 1999 to 2001.
It is not uncommon that discs are pressed at two or more plants simultaneously to get enough discs pressed in time. But the differences on the inserts suggests they were pressed at two different times - if done at the same time, it would make sense for Mars Colony to send the same master artwork to both plants.
Either way, it's no big mystery. I run a site about Nirvana releases, and we have three or four contenders for the very first pressing of the US CD release of Nevermind. In total there are tons of reissues, all with different matrix codes, and it's all a big giant mess. We're lucky with only two.
Not much more to do with this than to file it. I would very much appreciate high-quality, complete scans of front, back, and disc for my website, if you would be so kind.
Thanks again!