It looked from the photo`s as though there was a grey ring, possibly a lamination interface between inner and outer rings?
I have 60% finished the adapter now and went for a spherical seat and matching lapped knuckle. The four grub screws locking into the 45 deg. angular neck in, on the shaft above the knuckle and locking it down onto the seat. Will be modifying the end of the grub screws to a spherical face to make the adjustment a little smoother.
Gives me 1.5 to 2 deg. of axial movement.
Should be finished in the next few days.

How is your 4th axis progressing. I need shortly to put one on my old Seig X3. Did you go harmonic?