It’s this piece of super thin metal. The other one has more of the lip remaining. I looked at enough videos and photos of other people doing the spindle / axle rebuild and nobody else I saw has it at all. And nobody mentions it. But it’s in the parts diagrams of the front hub system. That’s the only reason I know it’s called the spindle seat.
My theory is that it helps to prevent the spindle welding itself to the knuckle.
In any case, I didn’t want this mostly deteriorated thing sitting in my setup so I just omitted it.
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