The more I think about it, the more I think RobS was right. The two piece shaft uses the circular "spring clip" for the best fit. The one piece shaft could eventually wear out and get loose. Maybe.

I think the shaft, which currently shows orange, will show as the color of the knob, just a tad darker. In the two piece shaft, it will be hard to notice the slightly lighter colored slot. I don't really see it while I'm driving and don't notice it when I'm stopped.

Oh yeah, and didn't people want the knob to stick out further? Currently it's about 1cm [edit: sorry, cm not mm] off the lens, but I could have it raised if people want.