ORIGINAL: Jezmo

The black ring Pe is referring to can best be seen with a coddington magnifier. (At least that's the best way without cutting the plug apart like Pe did here.) It has a built in light that can illuminate the very bottom of the insulator and a magnifier that allows a good view of the base of the insulator where the mixture ring will be seen.

P.S. A coddington magnifier is what a Doctor uses to look inside your ears and nose etc.