Severely worn impeller nut
 

Has anyone ever seen this?

Intake ducting was not dirty

Almost looks like balancing grinding, but odd with half the nut flats gone.

Noticed this while reassembling after full dlt with new up pipe and turbo back 4 to 5 inch exit
https://cimg4.ibsrv.net/gimg/www.die...cc180f4e4.jpeg
https://cimg6.ibsrv.net/gimg/www.die...38db1b0c3.jpeg
https://cimg2.ibsrv.net/gimg/www.die...96dc79c44.jpeg
https://cimg6.ibsrv.net/gimg/www.die...59fe49616.jpeg