Ahh yes, once more a pic provides brilliant illumination!
The different journal diameter is of no consequence, as far as mounting the clutch half goes. It's positively located rotationally/radially on the PTO by the key, and axially by the set screws.
The difference in the step diameter is due to the ball vs plain PTO side bearing - so that a normal-sized ball bearing can be used. For a given ID, a ball bearing will have a much higher load rating than a plain bush.
