Told you my eyes were crap, Lynn!
There is a central, cylindrical piece to the lower casting of the mount that bolts to the face of the carrier. Looks like it supports a pin/bolt assembly from the upper section of the mount. Cottered and flat washered at the very bottom. The entire upper section would pivot side to side until the beveled corners stopped any further movement. The last photo of that group shows the cylinder casting perfectly.
Ben mentioned earlier there is a rubber u-shaped pad bolted to the back of the mount that the Boys rests on when not in use. I can see the sleeve perhaps being used to lock the rifle from any elevation movement while the carrier is travelling, but from what is visible of the mount, would it not still swing in traverse unless something else secured it?
David