You can buy the shoulder, but you cannot use the knife, it is purely for looks.
They are part of the Security armour set. You unlock it at General rank and can see it at Brigadier. There's Left and Right. Left is mirrored and has no knife, and costs 70,000 credits. Right has the knife, and costs 270,000, presumably for being incredibly awesome.
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It should be standard on the right security shoulder.
it is on the chest piece of grenedier
after Halo reach,there is no more In total, there is :Halo 1,Halo 2, Halo 3, Halo ODST, Halo wars and Halo reach
No, Halo Reach shows what happened before Halo 1. In fact, in one of the missions in halo reach, you can see Master chief
You have hold RB (the punch button) on them when they are turned around (basically when you see their back go behind them and press RB).