Sandro,
It is surprising that anodizing was used on a moving part, as wear would eventually remove the coating over time. In present day mounts, I think these shafts are all stainless steel. The patterns you see can be explained by the way the shaft rides inside the bearings, as Peter has stated, with some slight gap between upper and lower bearings. But if you are not seeing any problems with shaft rotation and you don't have any issues with problematic periodic errors, you are probably OK.? ?
The clutch discs can be replaced as the nylon wears down over time. But after cleaning you might try sanding them down with some 220 mesh sandpaper to roughen the surface a little. But after 20 years you may wish to get some new ones. Thoroughly cleaning where the discs ride with some solvent should also help a bit.
Best regards,
John
