OK get ready
I think the base in this case might be called a "Traverse Wheel" (I could be corrected on this later)
The gun could be fired in 2 positions,
firstly in the "quick action" mode without the base.
Secondly (and primarily) when the gun was fired in the standard firing position the gun sat ON the base plate to stop the gun sinking into the earth from the recoil.
In this position the wheels sat on the outer rim of the Traverse Wheel (base).
The gun could also be rotated in different directions as the wheels could turn whilst sitting on this rim aiding movement for the crew.
..great weapon system, the biggest thing I ever fired, a remarkable experience!
hope this helps Al
Roy :)