Nuclear warhead transport container. The container would be taken to the assembly building and positioned behind the avionics section that make up the rear of the nose cone. The package then slides out on the two rails and can be attached to the nose cone.
Hatch that leads down to the "missile garage". Missiles would be lifted up through this hatch and positioned on the rails going left and right. When seen from above[b] the launch configuration is clear - two missiles to the right and one to the left, with the fourth being launched from the garage.