Ceramic Thrust "Bearings"

My "Space Age" thrust bushings arrived today, and are now installed, no thanks to the package, which was for the guide blocks, or the sticky over their instructions, recommending the site for bearings. Big thanks to Delta for their excellent manual.
As I was installing them, a question which has puzzled me for years recurred. If they mount the upper on an eccentric, so the blade rides in the edge, why don't they do the same for the lower thrust?
Moot now, because I changed the eccentric so the blade can't jump back into a careless gap left when setting the upper guide.
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