Thanks so much Dave for the detail you have gone to.Because of the careful casting and machining of the top bridge and lower bridge, the OD of the fork leg will not line up and easily slide up into both clamp and bridge unless these three pieces are undamaged. One leg inserted into place aligns the three parts of top bridge, lower clamp, and fork leg. (assuming the fork tubes are straight) The other leg then will slide up into place. The bridge and clamp bolts are torqued.
Now, the sliders are free to rotate 360 degrees on their respective legs but when we insert the axle from the left side into the left slider the slider will have it's flat inner surface square to the left side spacer in order to slide the axle through leg and spacer. Now the leg, axle, and spacer are still free to rotate on the leg but in order to slide the axle into the left side wheel bearing the leg must to rotated to the correct orientation for the axle to continue into the wheel bearing inner race. If the leg is not in the correct orientation the axle won't insert into the wheel. The left outer end (non threaded) of the axle is a precise fit into the left fork leg. This aligns that leg to wheel.
As the axle comes out of the right side of the wheel it passes through the right side spacer. The spacer's machined surfaces must be flat-to-flat for the axle to slide into the right fork slider. These machined surfaces align the right slider to the wheel. The axle bolt threads onto the right axle end, and it's machined OD fits into a precisely machined opening in the right slider. When the OD of the bolt is pulled down into the ID of the hole in the slider it aligns the right leg to the wheel. The only variable now is only if the fork legs are parallel to each other so we torque the right axle pinch bolts, get the bike down on it's wheels. Applying front brake we compress or bounce the forks a few times to allow the legs to parallel themselves before torqueing the left axle pinch bolts. If the legs are parallel, the outside of the axle will be flush with the machined outer surface of the left slider.
We have a saying in the UK "The penny has finally dropped".
Thanks also to all the contributors for their patience.
It doesn't pay to over-think some things!
Paul