Is that plug a platinum? If so, do a search on platinum plugs. Your picture shows very well, what some of us have tried to describe with words. As Doc said, plats do not work well in the older cars...stick with copper core plugs.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.