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Doing a steering knuckle service.
The king pin bearings and races are worn and will be replaced.
On disassembly I see the axle spindle has grooving.
What would cause this wear pattern?
Isn't there a void in that space between the wheel bearings?

One thing that I love about this forum is how much I learn from other people's projects. Although I cannot say that I know why that scoring exists, I am definitely as curious as you are!

Also, there is a lot of burnt where those races were

It is quite probable something jammed there and spun between the bearing race seats or a bearing race disintegrated to the point it almost fit in that space and moved there forcibly. Or it could just be John's answer below!

It is not uncommon for there to be a rough machined area between the two bearing seat locations. It is not actually "wear" that you are looking at.
John