Tonearm adjustments on the fly

That treatment is nothing more than anodizing, a fairly standard finish used to protect the aluminum surface from oxidation. It forms aluminum oxide on the surface and that is not electrically conductive. The thicker the layer the more insulating property it has. I've seen heatsinks that have been treated this way just for that purpose.Ever try drilling a hole through aluminum that has a fairly thick anodized surface? The drill will squeal and protest until it has broken through that hard layer. Aluminum oxide is also used in certain sand paper.

BillWojo

I've done that very thing! Had to grind the bit to give it a new edge. 

It forms aluminum oxide on the surface and that is not electrically conductive. The thicker the layer the more insulating property it has.

Exactly.