In an idle moment, I came across this movement shot in an Ebay listingPurchases made through these links may earn this site a commission from the eBay Partner Network for a 166.062. My immediate question is wtf has happened to the rotor?. Some sort of heat treatment? Bad lighting? Calling @Archer (or other knowledgeable types)

 

Looks like some sort of oxidation, you can see similar coloration on the main plate as well.

No idea what may have caused it.

 

It would seem there is a chance the watch had been worn without a gasket. Cleaning may or may not remove the disclouration.