MRL

Remember, however, the name of the armor. MJOLNIR. It could be a shortened code name for the armor itself.

MRL being coded name for MJOLNIR? Highly unlikely in my opinion. If it was the armor's code, it would likely be MJLNR or MLR rather that MRL. Also, another way to see this is using the debug camera mode available in Halo: Custom Edition, using the camera to go underground and looking up at the Chief's foot like that.