David you are right that it is the turret support. Now that it is almost finished It is very strong and rigid. I have cleaned and ground both parts and fitted the ball bearings and put the top on. It turns just so so easy. That means that I got it aligned properly. I guess the hold down brackets form the sideway forces plus the turret basket with ammo and two men on it would all add to the support. As far as the next models that had a bigger gun they must have done something to allow for recoil. The bearing support I made was a critical part as it would never have cut without it. All in all I am really happy how it has turned out.
Bob, my wife qualified a long time ago as a workshop assistant or was that me to her