ERIKS type GV (type C according to DIN) is equivalent to type M, but is a heavy-duty version with a double metal casing. This can be a useful solution with larger diameters in more demanding applications. There is also a version of this type with a dust lip; the GVST (type CS according to DIN).
The black color of these spark plugs is a result of carbon buildup, which is a common issue that can occur over time due to normal engine wear and tear. This carbon buildup can negatively impact the performance of the spark plug, leading to misfires, rough idling, and poor fuel efficiency. It is important to regularly inspect and replace black spark plugs to ensure optimal engine performance.