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Reversable hub. Great for dressing stuff like radii on the front where it's easy, then flip it over so the form is on the back.
Or like I had this week, a need to have a radius on both sides of the wheel to grind a 180 degree ball. Dress, flip, dress, both rads exactly the same.
I'm sure if you marked the spindle, and put it on the same spot it should repeat perfectly.
Of course, this blows away my invention idea. :(