you are right, in number 20 picture there is definitaly a bar passing in the wheel hole, but also some form of support beneath. From the picture it appear to be bolted or welded to the turret side. In number 23 picture it looks as it is roughly shaped to hold the wheel. My educated guess is that the thing beneath is a simple form of 'rack' (I am not finding a more appropriate word) that supports the wheel, while the bar in the center keep it in position. Obviously (at least in my mind) there must be also some for of restrain to avoid the wheel falling off, probably rope/chain/wire.