Cradle. Supports the piston, gun tube, and breech, and allows them to recoil during firing. The piston clamps to the breech via a series of bolts while supporting the gun tube at several locations.
A dampened recoil spring between the cradle and piston absorbs the gun's recoil energy.
The cradle connects to the tank turret through the mantlet which has a pair of trunnions supported by the turret and the elevation mechanism (a hydraulic actuator). The trunnions allow the cradle to rotate, controlled by elevation mechanism.