The V8 radiator filler plug. The shiny bit is a ring cut out of a section of old alloy vacuum cleaner nozzle which prevents the O-ring being pushed out from under the plug when the plug is tightened. The plastic plug (which some replace with brass fearing the plastic will fail in some way) also allowed me to fit a probe with a spade connection so I could monitor the coolant level. Originally fitted while I had a cooling system problem, after fixing that I've never been able to bring myself to remove it.