Nicholas,

The adjuster plug in question should be marked for Normal (for regular octane gasoline) or S for high test with a few values in between and on either side, I think. You have to pull the plug out and reinsert it so the correct octane value for the fuel you are using is selected. The plug assembly connects an array of resistors for each combination so that a specific resistance signal is sent to the Elecktronische Zuendleitung (EZL or electronic ignition control). You can artificially increase or decrease the setting by reinserting the plug for a higher or lower octane rating (higher octane equals advanced ignition timing). This subject is discussed in the Hot Rod section for tuning pretty frequently. US cars have a single resistor and they have to change the resistor while Euro cars have the plug you speak of. I hope this helps. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)