My hunch is the nut had to be on some sort of tripod or traverse and elevation device that a gun mounted to, but it's pretty big, so the tripod and gun would have been pretty big as well. That would fit with a military theme where they would specify all the tools to service an item like that are delivered with it.
The biggest weapon systems I've handled were M2 .50cal machine guns, and Mark 19 grenade launching machine guns, and they didn't have any hardware on them nearly that big. I'd guess that's getting into the anti-aircraft gun size stuff....maybe something mounted on a ship?