Steel wheel on the left, about 2" between the U-bolt and the weld for the bearing housing, wire wheel on the right, about 1". This also shows the effect of the different wheel offsets, and why the axles need to be different lengths as a result - most of the brake gear and the back-plate nuts are concealed in the well of the stud-mount wheel, whereas those with a wire wheel are clearly visible. Note this is a feature of the wheel offset, not the axle length. To see which axle you have you must measure the tube from U-bolt to weld as described.

April 2020: A measurement between the angled wire-wheel mounting surfaces on what is supposed to be a standard axle and hubs. Pointers of my tracking gauge in the middle of the surfaces:

1320 to a milli-metre or so:

All I need now is someone else to compare it with. Maybe next year. Or 2022 ...