Uuhhhh. These threads are always a pain because each time the answer is too long for me to really care. But since there's more than one of you expressing confusion... all right, hah.
When Halo 3 was released you had to get every achievement in the game for the Katana and Security Helmet armor permutations. In 2008 the Legendary Mappack was released. With this mappack came new achievements in addition to the originals. This caused an issue.
Due to the new achievements, suddenly the Katana and Security Helmet became glitched. To be perfectly honest? There is no requirement for it really. Or to be more accurate, the requirement varies from person to person. I have seen someone with as little as 360 Gamerscore with it, and people with 1650 Halo 3 gamerscore who have never had it unlock for them.
Getting them is more or less random at this point.
HOWEVER!!!...
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This is not the only issue with them. See, the people who have it unlock without the achievements that first shipped with the game (49 of them, which is why you'll sometimes see people say 'oh any 49 achievements to unlock them!'... That is a false statement) have their own issue. While it doesn't always happen, very frequently after unlocking, at some point the katana and security helmet with RElock themselves. The only way to prevent this from happening is by earning all 49 original Halo 3 achievements. That means every achievement that shipped with the game September 25th, 2007. That means any achievement that isn't a Vidmaster or requires a DLC map. The people who don't have the original 49 and haven't had it relock are just lucky. But the only way to ensure that it DOESN'T relock when you unlock them is by earning those original 49, as that is how you're actually supposed to get the armor permutations instead of just getting lucky with a glitch (though again, that same glitch oftentimes takes the armor permutations away again).