Just went thru the MB maint manual - 1981-1993 - no gapping info for plugs?? I must have obtained mine from the Haynes manual or the Ritter-Easley site back when I bought it.
The factory calls for what amts. to .032 for M103 plugs - not sure about an M116 engine. Perhaps one of the many 420 owners will provide that info soon.
The search facility should locate it for you.